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 = $$frac{P imes R imes T}{100}$$ If the amount is doubled then simple interest accumulated is Rs P => P = $$frac{P imes 25 imes T}{4 imes 100}$$ => T = $$frac{100 imes 4}{25}$$ => T = 4*4 = 16 years