# Compound Interest Using Formula Concise ICSE Class-9th

## Chapter/ Lesson / Exercise -3 Compound Interest (Using Formula) Selina Mathematics Solutions

**Compound Interest Using Formula** Concise ICSE Class-9th Mathematics Selina solutions Chapter-3 . We provide step by step Solutions of Exercise / lesson-3 **Compound Interest (Using Formula) ** for ICSE Class-9 Concise Selina Mathematics by RK Bansal.

Our Solutions contain all type Questions with Exe-3 A, Exe-3 B, Exe-3 C, Exe-3 D and Exe-3 E to develop skill and confidence. Visit official Website **CISCE** for detail information about ICSE Board Class-9 Mathematics .

**Compound Interest Using Formula** Concise ICSE Class-9th Mathematics Selina solutions Chapter-3

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### Exercise – 3(A) **Compound Interest (Using Formula**) Concise Selina Mathematics ICSE Class-9th

#### Question 1

Find the amount and the compound interest on Rs. 12,000 in 3 years at 5% compounded annually.

**Answer**

Given : P= Rs. 12,000; n = 3 years and r = 5%

= Rs. 13,891.50

C.I. = Rs. 13,891.50 – Rs. 12,000 = Rs. 1,891.50.

#### Question 2

Calculate the amount of Rs. 15,000 is lent at compound interest for 2 years and the rates for the successive years are 8% and 10% respectively.

**Answer**

**Given :** P = Rs. 15,000; n = 2 years; r_{1} = 8 % and r_{2} = 10%

= Rs. 17,820.

#### Question 3

**Answer **

Given : P = Rs. 6,000; n = 3 years; r_{1 }= 5%; r_{2 }= 8% and r_{3} = 10%

= Rs. 7,484.40

∴ C.I. = Rs. 7,484.40 – Rs. 6,000 = Rs. 1,484.40.

#### Question 4

What sum of money will amount to Rs. 5,445 in 2 years at 10% per annum compound interest ?

**Answer **

Given : P= Rs. 5,445 ; n = 2 years and r = 10%

⇒ Rs. 4,500

#### Question 5

On what sum of money will the compound interest for 2 years at 5% per annum amount to Rs. 768.75?

**Answer **

**Given :** C.I.= Rs. 768.75; n= 2 years and r = 5%

#### Question 6

Find the sum on which the compound interest for 3 years at 10% per annum amounts to Rs. 1,655.

**Answer **

**Given :** C.I. = Rs. 1,655; n = 3 years and r = 10%

#### Question 7

What principal will amount to Rs. 9,856 in two years, if the rates of interest for successive years are 10% and 12% respectively ?

**Answer **

**Given :** A = Rs. 9,856 ; n = 2 years ; r_{1} = 10 % and r_{2} = 12%

Question 8

On a certain sum, the compound interest in 2 years amounts to Rs. 4,240. If the rate of interest for the successive years is 10% and 15% respectively, find the sum.

**Answer **

‘

⇒ ( P + 4240 ) = P(1.265)

⇒ P = Rs. 16000

The sum is Rs.16,000.

#### Question 9

At what per cent per annum will Rs.6,000 amount to Rs.6,615 in 2 years when interest is compounded annually?

**Answer **

At 5% per annum the sum of Rs. 6,000 amounts to Rs. 6,615 in 2 years when the interest is compounded annually.

#### Question 10

At what rate per cent compound interest, does a sum of money become 1.44 times of itself in 2 years ?

**Answer **

Let Principal = Rs. y

Then Amount= Rs 1.44y

n= 2 years

On solving, we get

r = 20 %

#### Question 11

At what rate per cent will a sum of Rs. 4,000 yield Rs.1,324 as compound interest in 3 years ?

**Answer **

Given : P = Rs. 4,000, C.I. = Rs. 1,324 and n = 3 years

Now, A = P + I

⇒ A = Rs. ( 4,000 + 1,324 ) = Rs. 5,324

Thus, the rate of interest is 10%.

#### Question 12

(i) the rate of interest per annum.

(ii) the amount at the end of the third year.

**Answer **

**Given:** P = Rs. 5,000; A = Rs. 6,272 and n = 2 years.

= Rs. 7,024.64

#### Question 13

In how many years will Rs. 7,000 amount to Rs. 9,317 at 10% per annum compound interest ?

**Answer **

**Given :** P = Rs. 7,000; A = Rs. 9,317 and r = 10%.

On comparing,

n = 3 years

#### Question 14

Find the time, in years, in which Rs. 4,000 will produce Rs. 630.50 as compound interest at 5% compounded annually.

**Answer **

Given : P= Rs. 4,000; C.I.= Rs. 630.50 and r = 5%

On comparing,

n = 3 years

#### Question 15

Divide Rs. 28,730 between A and B so that when their shares are lent out at 10% compound interest compounded per year, the amount that A receives in 3 years is the same as what B receives in 5 years.

**Answer **

Let share of A = Rs. y

share of B = Rs (28,730 – y)

rate of interest= 10%

According to question,

Amount of A in 3 years= Amount of B in 5 years

Therefore share of A = Rs. 15,730

Share of B = Rs. 28,730 – Rs.15,730 = Rs. 13,000

#### Question 16

(i) What is the share of each out of Rs44,200 ?

**Answer **

**(i)** Let share of John = Rs y

share of Smith = Rs (44,200 – y)

rate of interest= 10%

According to question,

Amount of John in 4 years = Amount of Smith in 2 years

= Rs. 29,282

#### Question 17

The simple interest on a certain sum of money and at 10% per annum is Rs. 6,000 in 2 years, Find:

- the sum.
- the amount due to the end of 3 years and at the same rate of interest compounded annually.
- the compound interest earned in 3 years.

**Answer **

(i) I = Rs. 6,000, T = 2 years and R = 10%

= Rs. 39,930

(iii) C.I. earned in 3 years = A – P = Rs. (39,930 – 30,000) = Rs. 930.

#### Question 18

Find the difference between compound interest and simple interest on Rs. 8,000 in 2 years and at 5% per annum.

**Answer**

Given : P = Rs. 8,000, R = 5%, T = 2 years

For simple interest,

C.I. = A – P

= Rs. (8,820 – 8,000)

= Rs. 820

Now, C.I. – S.I. = Rs. ( 820 – 800 ) = Rs. 20.

Thus, the difference between the compound interest and the simple interest is Rs. 20.

**Compound Interest (Using Formula**) Exe-3 B, Concise Selina Mathematics ICSE Class-9th

#### Question 1

The difference between simple interest and compound interest on a certain sum is Rs. 54.40 for 2 years at 8 per cent per annum. Find the sum.

**Answer **

Let principal (P) = x

R = 8%

T = 2 years

X = Rs. 8500

Thus, principal sum = Rs. 8500

#### Question 2

A sum of money, invested at compound interest, amounts to Rs. 19,360 in 2 years and to Rs. 23,425.60 in 4 years. Find the rate per cent and the original sum of money.

**Answer **

(for 2 years) A = Rs. 19360

T = 2 years

Let P = X

X = Rs. 16,000

Thus, sum = Rs. 16000

#### Question 3

A sum of money lent out at C.I. at a certain rate per annum becomes three times of itself in 8 years. Find in how many years will the money becomes twenty-seven times of itself at the same rate of interest p.a.

**Answer **

Let principal = X, A = 3X, T = 8 years, R = ?

**Case I,**

T = 24

Time = 24 years.

#### Question 4

On what sum of money will compound interest (payable annually) for 2 years be the same as simple interest on Rs. 9,430 for 10 years, both at the rate of 5 per cent per annum ?

**Answer **

P = Rs. 9430

R = 5%

T = 10 years

Thus principal from = Rs. 46,000

#### Question 5

Kamal and Anand each lent the same sum of money for 2 years at 5% at simple interest and compound interst respectively. Anand recived Rs. 15 more than Kamal. Find the amount of money lent by each and the interest received.

**Answer **

Let principal = Rs. 100, R = 5% T = 2 years

= Rs. 615

#### Question 6

Simple interest on a sum of money for 2 years at 4% is Rs. 450. Find compound interest of the same sum and at the same rate for 2 years.

**Answer **

SI = Rs. 450

R = 4%

T = 2 years

P = ?

CI = A – P = 6084 – 5625 = Rs. 459

#### Question 7

Simple interest on a certain sum of money for 4 years at 4% per annum exceeds the compound interest on the same sum for 3 years at 5 per cent per annum by Rs. 228. Find the sum.

**Answer **

Let principal (P), R = 4%, T = 4 years

P = Rs. 96000

Thus, Principal = Rs. 96000

#### Question 8

Compound interest on a certain sum of money at 5% per annum for two years is Rs. 246. Calculate simple interest on the same sum for 3 years at 6% per annum.

**Answer **

CI = Rs. 246, R = 5%, T = 2 years

CI = A – P

Question 9

A certain sum of money amounts to Rs. 23,400 in 3 years at 10% per annum simple interest. Find the amount of the same sum in 2 years and at 10% p.a. compound interest.

**Answer **

Let the sum (principle) = X

Given Amount = 23400, R = 10% and T = 3 years

A = 21780

The amount of the same sum in 2 years and at 10% p.a. compound interest is 21780.

** **Question 10

Mohit borrowed a certain sum at 5% per annum compound interest and cleared this loan by paying Rs. 12,600 at the end of the first year and Rs. 17,640 at the end of the second year. Find the sum borrowed.

**Answer **

For the payment of Rs. 12,600 at the end of first year :

A = Rs. 12,600; n = 1 year and r = 5%

∴ Sum borrowed = Rs. (12,000 + 16,000 ) = Rs. 28,000.

### Concise Selina Mathematics ICSE Class-9th **Exe-3 C**, **Compound Interest (Using Formula**)

** **Question 1

If the interest is compounded half-yearly, calculate the amount when principal is Rs. 7,400; the rate of interest is 5% per annum and the duration is one year.

**Answer **

Given: P = Rs. 7,400; r = 5% p.a. and n = 1 year

Since the interest is compounded half-yearly,

** **

= Rs. 7,774.63

#### Question 2

Find the difference between the compound interest compounded yearly and half-yearly on Rs. 10,000 for 18 months at 10% per annum.

**Answer **

(i) When interest is compounded yearly :

Given : P = Rs. 10,000 ; n = 18 months = year and r = 10% p.a.

For 1 year

** **

= Rs. 11,576.25

C.I.= Rs.11,576.25 – Rs.10,000 = Rs. 1,576.25

Difference between both C.I. = Rs. 1,576.25 – Rs. 1,550 = Rs. 26.25

#### Question 3

A man borrowed Rs.16,000 for 3 years under the following terms:

20% simple interest for the first 2 years.

20% C.I. for the remaining one year on the amount due after 2 years, the interest being compounded half-yearly.

Find the total amount to be paid at the end of the three years.

**Answer **

For the first 2 years

** **

⇒ A = 27,104

The total amount to be paid at the end of the three years is Rs. 27,104.

#### Question 4

What sum of money will amount to Rs. 27,783 in one and a half years at 10% per annum compounded half yearly ?

**Answer **

** **

⇒ P = 24,000

The sum of Rs. 24,000 amount Rs. 27,783 in one and a half years at 10% per annum compounded half yearly.

#### Question 5

Ashok invests a certain sum of money at 20% per annum, compounded yearly. Geeta invests an equal amount of money at the same rate of interest per annum compounded half-yearly. If Geeta gets Rs. 33 more than Ashok in 18 months, calculate the money invested.

**Answer **

**(i) For Ashok (interest is compounded yearly) : **

Let P = Rs. y; n = 18 months = year and r = 20% p.a.

**For 1 year**

Money invested by each person=Rs. 3,000.

#### Question 6

At what rate of interest per annum will a sum of Rs. 62,500 earn a compound interest of Rs. 5,100 in one year? The interest is to be compounded half yearly.

**Answer **

** **

⇒ r = 8

The rate of interest is 8%.

#### Question 7

In what time will Rs. 1,500 yield Rs. 496.50 as compound interest at 20% per year compounded half-yearly ?

**Answer **

Given: P=Rs. 1,500; C.I.= Rs. 496.50 and r = 20%

Since interest is compounded semi-annually

** **

** **Question 8

Calculate the C.I. on Rs. 3,500 at 6% per annum for 3 years, the interest being compounded half-yearly.

Do not use mathematical tables. Use the necessary information from the following:

(1.06)^{3} =1.191016; (1.03)^{3} = 1.092727

(1.06)^{6} =1.418519; (1.03)^{6} = 1.194052

**Answer **

**Given** **:** P = Rs. 3,500; r = 6% and n = 3 years

Since interest is being compounded half-yearly

** **

= 3,500[(1.03)^{6} – 1 ]

= 3,500[ 1.194052 – 1 ]

= 3,500 x 0.194052

= Rs. 679.18

#### Question 9

Find the difference between compound interest and simple interest on Rs.12,000 and in years at 10% compounded yearly.

**Answer **

** **

= Rs. 13,860

∴ C.I. = Rs. 13,860 – Rs. 12,000 = Rs. 1,860

∴Difference between C.I. and S.I.

= Rs. 1,860 – Rs. 1,800 = Rs. 60.

#### Question 10

Find the difference between compound interest and simple interest on Rs. 12,000 and in years at 10% compounded half-yearly.

**Answer 10**

A = Rs. 13,891.50

C.I. = Rs. 13,891.50 – Rs. 12,000 = Rs. 1,891.50

∴ Difference between C.I. and S.I = Rs. 1,891.50 – Rs. 1,800 = Rs. 91.50.

**EXERCISE- 3(D),** **Compound Interest (Using Formula**)Selina solutions for ICSE Class -9th

Question 1

The cost of a machine is supposed to depreciate each year at 12% of its value at the beginning of the year. If the machine is valued at Rs. 44,000 at the beginning of 2008, find its value :

(i) at the end of 2009.

(ii) at the beginning of 2007.

**Answer **

Cost of machine in 2008 = Rs. 44,000

Depreciation rate = 12%

(i) ∴ Cost of machine at the end of 2009

Question 2

The value of an article decreases for two years at the rate of 10% per year and then in the third year it increases by 10%. Find the original value of the article, if its value at the end of 3 years is Rs. 40,095.

**Answer **

Let X be the value of the article.

The value of an article decreases for two years at the rate of 10% per year.

The value of the article at the end of the 1^{st} year is

X – 10% of X = 0.90X

The value of the article at the end of the 2^{nd} year is

0.90X – 10% of (0.90X) = 0.81X

The value of the article increases in the 3^{rd} year by 10%.

The value of the article at the end of 3^{rd } year is

0.81x + 10% of (0.81x) = 0.891x

The value of the article at the end of 3 years is Rs. 40,095.

0.891X = 40,095

⇒ X = 45,000

The original value of the article is Rs. 45,000.

#### Question 3

According to a census taken towards the end of the year 2009, the population of a rural town was found to be 64,000. The census authority also found that the population of this particular town had a growth of 5% per annum. In how many years after 2009 did the population of this town reach 74,088 ?

**Answer **

Population in 2009 (P) = 64,000

Let after n years its population be 74,088(A)

Growth rate= 5% per annum

On comparing, we get,

n = 3 years

#### Question 4

The population of a town decreased by 12% during 1998 and then increased by 8% during 1999. Find the population of the town, at the beginning of 1998, if at the end of 1999 its population was 2,85,120.

**Answer **

Let the population in the beginning of 1998 = P

The population at the end of 1999 = 2,85,120(A)

r_{1} = – 12% and r_{2} = + 8%

Question 5

A sum of money, invested at compound interest, amounts to Rs. 16,500 in 1 year and to Rs. 19,965 in 3 years. Find the rate per cent and the original sum of money invested.

**Answer **

Let sum of money be Rs P and rate of interest = r %

Money after 1 year = Rs. 16,500

Money after 3 years = Rs. 19,965

**For 1 year**

Question 6

The difference between C.I. and S.I. on Rs. 7,500 for two years is Rs. 12 at the same rate of interest per annum. Find the rate of interest.

**Answer **

**Given :** P = Rs. 7,500 and Time(n) = 2 years

Let rate of interest = y%

⇒ y^{2} = 16

⇒ y = 4 %

#### Question 7

A sum of money lent out at C.I. at a certain rate per annum becomes three times of itself in 10 years. Find in how many years will the money become twenty-seven times of itself at the same rate of interest p.a.

**Answer **

Let Principal be Rs y and rate= r%

According to 1^{st} condition

Amount in 10 years = Rs 3

On comparing, we get

n = 10 x 3 = 30 years

#### Question 8

Mr. Sharma borrowed a certain sum of money at 10% per annum compounded annually. If by paying Rs.19,360 at the end of the second year and Rs. 31,944 at the end of the third year he clears the debt; find the sum borrowed by him.

**Answer **

At the end of the two years the amount is

⇒ P = Rs. 40,000

Mr. Sharma borrowed Rs. 40,000.

#### Question 9

The difference between compound interest for a year payable half-yearly and simple interest on a certain sum of money lent out at 10% for a year is Rs. 15. Find the sum of money lent out.

**Answer **

Let sum of money be Rs. y

To calculate S.I.

⇒ `y/400 = 15 ⇒ y = Rs. 6,000.

#### Question 10

The ages of Pramod and Rohit are 16 years and 18 years respectively. In what ratio must they invest money at 5% p.a. compounded yearly so that both get the same sum on attaining the age of 25 years?

**Answer **

Let Rs.X and Rs.Y be the money invested by Pramod and Rohit respectively such that they will get the same sum on attaining the age of 25 years.

Pramod will attain the age of 25 years after 25 – 16 = 9 years

Rohit will attain the age of 25 years after 25 -18 = 7 years

Pramod and Rohit should invest in 400 : 441 ratio respectively such that they will get the same sum on attaining the age of 25 years.

### Exercise – 3(E)

Simple interest on a sum of money for 2 years at 4% is Rs .450. Find compound interest on the same sum and at the same rate for 1 year, if the interest is reckoned half yearly.

**Answer **

**1 ^{st} case
**Given : S.I. = Rs 450 ; Time = 2 years and Rate = 4%

= Rs. 5852.25

∴ C.I. = 5,852.25 – 5,625 = Rs. 227.25

#### Question 2

Find the compound interest to the nearest rupee on Rs. 10,800 for years at 10% per annum.

**Answer **

Given : P = Rs. 10,800 ; Time = years and Rate = 10% p.a.

For 2 years

∴ Rs.13,721 – Rs.10,800 = Rs.2,921

#### Question 3

The value of a machine, purchased two years ago, depreciates at the annual rate of 10%. If its present value is Rs.97,200, find:

- Its value after 2 years.
- Its value when it was purchased.

**Answer **

(i) Present value of machine(P) = Rs.97,200

Depreciation rate = 10%

#### Question 4

Anuj and Rajesh each lent the same sum of money for 2 years at 8% simple interest and compound interest respectively. Rajesh received Rs. 64 more than Anuj. Find the money lent by each and interest received.

**Answer **

Let the sum of money lent by both Rs. y

For Anuj

P = Rs.y ; rate = 8% and time = 2 years

#### Question 5

Calculate the sum of money on which the compound interest (payable annually) for 2 years be four times the simple interest on Rs. 4,715 for 5 years, both at the rate of 5% per annum.

**Answer **

**Given :** Principal = Rs.4,715; time = 5 years and rate= 5% p.a.

#### Question 6

A sum of money was invested for 3 years, interest being compounded annually. The rates for successive years were 10%, 15% and 18% respectively. If the compound interest for the second year amounted to Rs. 4,950, find the sum invested.

**Answer **

**Given :** C.I. for the 2^{nd} year = Rs. 4,950 and rate = 15%

The sum invested is Rs.30,000.

#### Question 7 Compound Interest Using Formula

A sum of money is invested at 10% per annum compounded half yearly. If the difference of amounts at the end of 6 months and 12 months is Rs.189, find the sum of money invested.

**Answer **

Let the sum of money be Rs. y

and rate = 10% p.a. compounded half yearly

y = 3600.

#### Question 8

Rohit borrows Rs. 86,000 from Arun for two years at 5% per annum simple interest. He immediately lends out this money to Akshay at 5% compound interest compounded annually for the same period. Calculate Rohit’s profit in the transaction at the end of two years.

**Answer **

P = Rs. 86,000; time = 2 years and rate = 5% p.a.

**To calculate S.I**

Profit = C.I. – S.I. = Rs.8,815 – Rs.8,600 = Rs.215

#### Question 9

The simple interest on a certain sum of money for 3 years at 5% per annum is Rs.1,200. Find the amount and the compound interest due on this sum of money at the same rate and after 2 years. Interest is reckoned annually.

**Answer **

Let Rs.X be the sum of money.

Rate = 5 % p.a. Simple interest = Rs.1,200, n = 3 years.

⇒ A = 8,000( 1.1025 )

⇒ A = 8,820

C.I. = A – P

⇒ C.I. = 8,820 – 8,000

⇒ C.I. = 820.

The amount due after 2 years is Rs. 8,820 and the compound interest is Rs. 820.

#### Question 10

Nikita invests Rs.6,000 for two years at a certain rate of interest compounded annually. At the end of first year it amounts to Rs.6,720. Calculate:

(a) The rate of interest.

(b) The amount at the end of the second year.

**Answer **

Let X % be the rate of interest.

P = Rs. 6,000, n = 2 years, A = Rs.6,720

For the first year

⇒ A = 7,526.40

The amount at the end of the second year = Rs. 7,526.40

End of **Compound Interest Using Formula**** **Solutions :–

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