If a sum of money amounts to 6 times of itself in 25 years at simple interest, then what is the rate of interest per annum?

20%

A sum of money amounts to 6 times of itself in 25 years at simple interest.

Let sum of money = P . So , Amount = 6P 

Amount = Principal + Interest

6P = P + Interest

Interest = 6P - P = 5P

Simple Interest = \(\frac{Principal ×Rate × Time }{100}\)

5P = \(\frac{P ×Rate × 25 }{100}\)

Rate = 20%