A sum of money doubles itself in 10 years. How many years will it take to triple itself?

Let P = 100,
A = 200,
It means I = 100

Now We want A = 300,
means I = 200
Years for I of Tk. 100 = 10
Years for I of Tk. 200 = (10/100) × 200 = 20 Years

Alternative Method:
Assuming simple interest, let the amount be P and rate of interest per annum be R%.

Given that P becomes 2P in 10 years.
=> P + PX10X R/100 = 2P
=> 1 + R/10 = 2
=> R = 10

Let P triple itself in N years:
=> P + PXNX 10/100 = 3P
=> 1 + N/10 = 3
=> N = 20 years