If simple interest on a certain sum of money is Tk. 400 and the rate of interest per annum equals the number of years, then the rate of interest is-

Question: If simple interest on a certain sum of money is Tk. 400 and the rate of interest per annum equals the number of years, then the rate of interest is-

Solution:
Let,
The rate of interest is x%
∴ Number of years is x

The interest of 1 year of 100 taka is x
∴ The interest of x years of 100 Taka is x × x = x²

ATQ,
x² = 400
∴ x = 20

∴ The rate of interest is 20%