In certain years a sum of money is doubled to itself at 6 1/4 % simple interest per annum, then the required time will be

let the principal amount be Rs P and rate if interest be R% per annum

It is given that R = $$\frac{25}{4}$$% when interest is Simple Interest

Let T be time to double up the principal amount with the given rate of interest

SI = $$\frac{P \times R \times T}{100}$$

If the amount is doubled then simple interest accumulated is Rs P

=> P = $$\frac{P \times 25 \times T}{4 \times 100}$$

=> T = $$\frac{100 \times 4}{25}$$

=> T = 4*4 = 16 years