Question

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

Answer 1

Amount a (1st yr)= 16500
Amount c (3rd yr)= 19965.
n= no of years

by the formula:
Amount a =P(1+r/100)ⁿ
1st year:
16500=P(1+r/100)¹
and 3rd year:
19965=P(1+r/100)³

Dividing both the equations; 
19965/16500=P(1+r/100)³ / P(1+r/100)¹

121/100= (1+r/100)²
taking under root
11/10= 1 + r/100
1/10 x 100 = r
r=10%
using the formula again we will get the original sum = 15000

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