A certain sum of money yields Tk. 1261 as compound interest for three years at 5% per annum. The sum is ?

Question: A certain sum of money yields Tk. 1261 as compound interest for three years at 5% per annum. The sum is ?

Solution:
Let the principal be Tk. P

Now,
1261 = P(1 + r)n - P
1261 = P(1 + 5/100)3 - P
1261 = P(105/100)3 - P
1261 = P(21/20)3 - P
1261 = P(9261/8000 - 1)
1261 = P (1261/8000)
1 = P/8000
P = 8000