A sum of money invested at compound interest doubles itself in 6 years. At the same rate of interest it will amount to eight times of itself in :
Options:
A . 10 years
B . 15 years
C . 18 years
D . 12 years
Answer: Option C Answer: (c)Let the sum be x. Then,$2x = x(1 + r/100)^6$2 = $(1 + r/100)^6$Cubing both sides,8 =$((1 + r/100)^6)^3$8 = $(1 + r/100)^18$$8x = x(1 + r/100)^18$The sum will be 8 times in 18 years.i.e., Time = 18 yearsUsing Rule 5,Here, m = 2, t = 6 yearsIt will becomes 8 times of itself= $2^3$ times of it selfin t × n years = 6 × 3 = 18 years
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