If a certain sum becomes two times in 7 years at compound interest, then in how many years, it will become eight times ?

Answer : Option B

Explanation :

$A=Pleft(1+frac{R}{100} ight)^{T}$ $Rightarrow 2=1left(1+frac{mathrm{R}}{100} ight)^{7}$ On cubing both sides $2^{3}=8=left(1+frac{mathrm{R}}{100} ight)^{21}$ ∴ Required time = 21 years Alternative: