1. Compound Inverse problems

2. We have already learnt the formula,
where four variables A, P, r and n are involved. Out of these four
variables, if any three variables are known, then we can calculate
the fourth variable.

3. when interest is being compounded annually?
Example 1: At what rate per annum will Rs. 640 amount to Rs in 2 years,
when interest is being compounded annually?
Solution: Given: P = Rs. 640, A = Rs , n = 2 years
To Find: Rate(r)
We know,
To remove decimal point from the
numerator, we multiply both
numerator and denominator by 100
= x 100 =77440
640=640 x 100 = 64000
121
968
800
100

4. 121 = 11 x 11
100 = 10 x 10
since powers are same
we compare the base
Ans:- Rate r = 10% per annum

5. per annum compound interest.
Example 2 : In how much time will a sum of Rs.1600 amount to Rs at 5%
per annum compound interest.
Solution Given: P = Rs. 1600, A = Rs , r = 5% per annum
To Find :- No of years(n)
We know 21
To remove decimal point from the
numerator, we multiply both
numerator and denominator by 100
= x 100 =185220
160=160 x 100 = 16000 20
9261
8000

6. Ans:- No of years is 3 years
9261 = 21 x 21 x 21
8000 = 10 x 10 x 10
since bases are same
we compare the powers
∴ n= 3 years
Ans:- No of years is 3 years

7. Try these In how much time will a sum of Rs.400 amount to Rs. 529
at 15 % per annum compound interest.