How To Solve Compound And Simple Interest Problems Quickly

Question 4. The compound interest on Rs. 30,000 at 7% per annum is Rs. 4347. The period (in years) is:
                   A. 2
                   B.1/2
                   C. 3
                   D.4

Solution-  Amount = Rs. (30000 + 4347) = Rs. 34347.

                   Let the time be n years.

                   then,30000 \left ( 1+\frac{7}{100} \right )^n = 34347

\left ( \frac{107}{100} \right )^n = \frac{34347}{30000}

\left ( \frac{107}{100} \right )^2

Correct option: A

Question 5. There is 60% increase in an amount in 6 years at simple interest. What will be the compound interest of Rs. 12,000 after 3 years at the same rate?

                A.Rs. 2160

                B.Rs. 3120

                C.Rs. 3972

                D.Rs. 6240

Solution 

                 Let P = Rs. 100. Then, S.I. Rs. 60 and T = 6 years.

                 R= \left ( \frac{100\times60}{10\times6} \right )= 10% p.a.
                 Now, P = Rs. 12000. T = 3 years and R = 10% p.a

                 C.I = Rs \left [ 12000\times\left \{ \left ( 1+\frac{10}{100} \right )^3-1 \right \} \right ]
                 =Rs\left ( 12000\times\frac{331}{1000} \right )

                =3972