How To Solve Compound And Simple Interest Problems Quickly

Type 2: Solve Simple Interest and Compound Interest Quickly.  Find the amount/time/rate of interest when CI or SI or their difference is given

Question 1. The difference between the CI and SI on a certain amount is at 10% p.a. for 3 years is Rs. 31. Find the principal?

Options

A. 1000

B. 3100

C. 310

D. 100

Solution    The difference between compound interest and simple interest for three years is 31.

On solving further, we get

P = 1000

Correct option: A

Question 2. If the SI on a sum of money for 2 years at 5% p.a. is Rs. 500, what is the CI on the same sum at the same rate and for the same time?

Options

A. Rs. 512.5

B. Rs 521.5

C. Rs 515.2

D. Rs 215.5

Solution      Sum = \frac{500 × 100 }{2 × 5}

Sum = \frac{50000 }{10}

Sum = 5000

Amount =  5000 (1+ \frac{5}{100})^{2}

5000 × 1.05 × 1.05 = 5512.5

CI = 5512.5 – 5000

CI = Rs. 512.5

Correct option: A

Question 3. The difference between CI and SI on a principal of Rs. 15,000 for two years is Rs. 24. What is the annual rate of interest?

Options

A. 16%

B. 4%

C. 8%

D. 6%

Solution      CI – SI = \frac{P × (R)^2}{(100)^2}

On solving further we get,

24 × (100)^{2}= 15000 × R²

R² = 16

R = 4%

Correct option: B