A sum of money placed at compound interest doubles itself in 5 years in how many years it would to 8 times of itself at the same rate of interest?
Options:
A . 7 years
B . 10 years
C . 15 years
D . 20 years
Answer: Option C
Let principal be Rs x and the rate is R % p.a. Then X × (1 + R/100)5 = 2x => (1 + R/100) 5 = 2 Let x × (1 + R/100) t = 8x => (1 + R/100) t = 8 = 23 ={(1 + R/100) 5 }3 => (1 + R/100) t = (1 + R/100) 15 => T = 15 Years
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