If in 13 years a fixed sum doubles at simple interest, what will be the interest rate per year? (Correct to two decimal places)

7.69%

Given :-

In 13 years fixed sum doubles itself at simple interest.

Amount = Principal + Interest

Let fixed sum = P and final amount after 13 years = 2P

SI = 2P - P = P

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

P = \(\frac{P ×Rate × 13 }{100}\)

Rate = \(\frac{P × 100 }{P×13}\)

= 7.69%