A certain sum amounts to Rs. 7000 in 2 years and to Rs. 8000 in 3 years. Find the sum ?

Amount = Rs. 7000, Time = 2 years
Amount = Rs. 8000, Time = 3 years
To calculate sum, we must first calculate rate of interest.
Simple interest = PRN / 100

S.I. on Rs. 7000 for 1 year = Rs. (8000 – 7000) = Rs. 1000
1000 = 7000 × R × 1 / 100
R = 11 23/7%
Let sum be Rs. X
7000 = P [ 1 + ( 100 / 7 × 100 ) ]²
8 / 7 × 8 / 7 × P = 7000
P = 5359.37