# How To Solve Compound Interest Problems Quickly

## How To Solve Quickly Compound Interest & Definition of Compound Interest

Compound interest is the interest calculated on the original principal and on the accumulated past interest of a deposit or loan.

• Compound interest calculated by multiplying the original principal amount one plus the annual interest rate raised to the number of compound periods minus one.

Basic Formula for the Compound Interest is

A= P(1 + r/n)nt

Where, A = Amount
P = Principal
r = Interest rate(decimal)
n = number of times interest is compounded per unit ‘t’ ## Type 1: How To Solve Compound Interest Quickly (Yearly, Quarterly, and Half-yearly)

### Ques 1.

If a sum of Rs.100 is invested for 10% p.a at CI then the sum of the amount will be Rs.121 in

Options:

A. 2 years

B.1 year

C.1.5 years

D. 3 years

#### Solution:

We know that, Amount = P (1+r/100)^t

121 = 100 (1 + 10/100)t

121/100 = (11/10) t

(11/10)2 = (11/10) t

t = 2 years

### Ques 2.

Find the CI on Rs. 10,000 in 2 years at 2 % per annum, the interest being compounded half-yearly.

Options:

A. 408

B. 406.04

C. 409.03

D. 405.50

#### Solution:

According to the question

P =10000, r = 2%, t = 2

We know that,

Amount = P (1+r/2/100)2t

A = 10000 (1+ 2/2/100)4

A = 10000 (1+ 1/100)4

A = 10000 (1+ 0.01)4

A = 10000 (1.01)4

A = 10000 (1.04)

A = 10406.04

Now, CI = A – P

CI = 10406.04 – 10000

CI = 406.04

### Ques 3.

Find the CI on Rs. 2000 in 9 months at 12% p.a. if the interest is calculated quarterly.

Options:

A. 251.01

B. 251

C. 304

D.185.45

#### Solution:

According to the question,

P = 2000, r = 6%, t = 9 months (3 quarter)

We know that,

Amount = P (1+r/4/100)4t

A = 2000 (1+12/4/100)3

A = 2000 (1+3/100)3

A = 2000 (1+0.03)3

A = 2000 (1.03) 3

A = 2000 (1.09)

A = 2185.45

Now, CI = A – P

CI = 2185.45– 2000 = 185.45

## Type 2: Solve Compound Interest Quickly (Rate of Interest, Time period, Principal)

### Ques 1.

The CI on Rs. 20,000 at 6% per annum is Rs. 2472. Find the period (in years):
Options:

Options:

A. 2

B. 4

C. 5

D. 3

#### Solution:

Amount = 20000 + 2472 = Rs. 22472

Let the time = n years

So, 20000 (1 + 6/100)n = 22472

(106/100)n = 22472/20000 = 11236/10000 = (106/100)2

Therefore, n = 2 years

### Ques 2.

The principal amount is put on CI for two years at 40%. It gets 964 more if the interest is payable half yearly. Calculate the sum.
Options:

A. Rs 8485
B. Rs 8485.91
C. Rs 8480
D. Rs 8455.91

#### Solution:

Let us assume the Principal as Rs. 100
When compounded annually
A = 100 [1+40/100]^2
A= 196
When compounded half yearly
A = 100[1+40/2/100]^4
A = 100[1+20/100]^4
A = 207.36
Difference, 196 – 144 = 11.36
If difference is 11.36, then Principal = Rs 100
If difference is 964, then Principal = 100/11.36 × 964
P = Rs 8485.91

### Ques 3.

In what time the CI on Rs 800 at 30% pa will amount to Rs.1352 if calculated annually?

Options:

A. 3 years
B. 1.6 years
C. 2 years
D. 5 years

#### Solution:

We know that,
Amount = P (1+r/100)^t
1352 = 800 (1 + 30/100)^t
1352/800 = (1.3)^t
1.69 = (1.3)^t
1.69 = (1.3)^2
Therefore, t = 2 years

## Type 3: Find sum or rate of interest when compound interest and S.I are given (How To Solve C.I Quickly)

### Question 1.

If the difference between compound interest and simple interest on a certain principal amount is Rs 500 at 10% p.a. for 2 years. Then calculate the sum.

Options:

A. 45000
B. 50000
C. 55000
D. 5000

#### Solution:

Given the difference between SI and CI = 500

Time = 2 years

When the difference between SI and CI is of two years then,
Difference = P(R)²/(100)²
Where P = principal amount, R = rate of interest

According to the question:

500 = P(10^2)/100^2

P = 50,000

### Question 2.

The difference between CI and SI on an amount of Rs. 15,000 for 2 years is Rs. 96. Find the rate of interest?
Options:

A. 8%
B. 5%
C. 4%
D. 3%

#### Solution:

Difference = P(R^2)/100^2

96 = 15000(R^2)/10000

(96*10000)/15000 = R^2

R^2 = 64

R = 8%

### Question 3.

The difference between CI and SI calculated annually on a certain amount of money for two years at 4% pa is Re. 1. Find the sum:
Options:

A. 600
B. 700
C. 650
D. 625

#### Solution:

When the difference between SI and CI is of two years then,
Difference = P(R)²/(100)²
Where P = principal amount, R = rate of interest

1 = P(4^2)/100^2

P = 625

## Type 4: When sum of money becomes x time in ‘a’ years and y times in ‘b’ years. (Solve Compound Interest Quickly)

### Question 1.

A sum of money borrowed under CI gets double in 5 years. When will it become eight times of itself if the rate of interest remains same?
Options:

A. 15 years
B. 20 years
C. 10 years
D. 5 years

#### Solution:

We know that,
x^(1/a) = y^(1/b)
2^(1/5) = 8^(1/b)
2^(1/5) = 2^(3/x)
1/5 = 3/x
x = 15 years

### Question 2.

If a certain amount of sum becomes 9 times in 2 years at compound interest, then find out the rate of interest?
Options:

A. 250%
B. 100%
C. 200%
D. 20%

#### Solution:

We know that,
r = 100 [(A/P)^(1/t) – 1)]
Therefore,
r = 100 (9(1/2) -1)

r = 100 (3-1)= 100*2
r = 200%

### Question 3.

At what rate percentage will certain amount of money become 8 times in three years?
Options:

A. 113. 79%
B. 100%
C. 110. 79%
D. 130%

#### Solution:

We know that,
r = 100 [(A/P)^(1/t) – 1)
r = 100 (8^(1/3) – 1)
r = 100 * (2-1)
r = 100%

## Type 5: When rates are different for different years (How To solve Compound Interest Quickly)

### Question 1.

Ravi took an amount of Rs.20000 as loan at CI charging 5%, 10% and 20% for the 1st year, 2nd year, and 3rd year respectively. Find out the total interest to be paid by Ravi at the end of the 3rd year?
Options:

A. Rs. 7270

B. Rs. 2270

C. Rs. 7720

D. Rs. 7027

#### Solution:

According to the question,

P = 20000

R = 5%, 10%, and 20%

T= 1, 2, and 3 years

Amount = 20000 [(1+5)/100 * (1+10)/100 * (1+20)/100]

Amount = 20000 [(105/100) * (110/100) * (120/100)]

Amount = 20000 * 1.05 * 1.1 * 1.2

Amount = 27720

We know that, CI = A – P

CI = 27720 – 20000

CI = Rs. 7720

### Question 2.

If the rate of CI for the 1st year is 8%, 2nd year is 10%, and for 3rd year is 15% then find the amount and the CI on Rs 10000 in three years.

Options:

A. 3626

B. 6236

C. 3666

D. 3662

#### Solution:

We know that, Amount = P (1+r1/100) (1+r2/100) (1+r3/100)

Amount = 10000 (1+8/100) (1+10/100) (1+15/100)

Amount = 10000 (1.08) (1.1) (1.15)

Amount = 13662

Now, CI = amount –principal

CI = 13662 – 10000

CI = 3662

### Question 3.

Karan bought a bike of Rs. 60000 by paying cash. He borrowed the cash from his friend at rate of interest 5% for the 1st year and 15% for the 2nd year. Find out the total amount he has to pay after 2 years to his friend.

Options:

A. Rs. 72450

B. Rs. 74250

C. Rs. 72540

D. Rs. 75420

#### Solution

We know that, Amount = P (1+r1/100) (1+r2/100)

Amount after two years = 60000 (1 + 5/100) (1 + 15/100)

Amount after two years = 60000 * 1.05 * 1.15

Amount after two years = Rs. 72450