A certain sum becomes 4 times in 7 years, at simple interest, then in how many years it will become 16 times?

∴ Interest earned in 7 years = Thrice the principal ∴ Interest earned in 35 years = Fifteen times the principal ∴ Principal will become 16 times of itself in 35 years Alternative : ∴ Amount = Principal + Interest In 1st Case $4 imes P+frac{P imes R imes 7}{100}$ Rate $=frac{300}{7}$ In 2nd case $16 imes mathrm{P}=(mathrm{P}) frac{mathrm{P} imes mathrm{R} imes mathrm{T}}{100}$ $15 imes mathrm{P}=mathrm{P} imes frac{300 imes mathrm{T}}{7 imes 100}$ Time = 35 years Alternative : S.I. = 400 – 10 = 300 ......(i) = 1600 – 100 = 1500 ......(ii) From equation (i) & (ii),