A sum of money placed at compound interest doubles itself in 5 years. In how many years, it would amount to eight times of itself at the same rate of interest ?
Options:
A . 20 years
B . 10 years
C . 7 years
D . 15 years
Answer: Option D Answer: (d)Let the Principal be P and rate of interest be r%.2 P = P$(1 + r/100)^2$2 = $(1 + r/100)^5$ ...(i)On cubing both sides,8 = $(1 + r/100)^15$Time = 15 yearsUsing Rule 5,Here, m = 2, t = 5 yearsIt becomes 8 times = $2^3$ timesin t × n = 5 × 3 = 15 years
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