A sum of money doubles itself at compound interest in 15 years.in how many years will it become eight times?

P(1+R/100)¹⁵=2P

(1+R/100)¹⁵=2P/P=2

LET P(1+R/100)ⁿ=8P

(1+R/100)ⁿ=8=2³={(1+R/100)¹⁵}³[USING (I)]

(1+R/100)^N=(1+R/100)⁴⁵

n=45.

Thus, the required time=45 years.