A sum borrowed under compound interest doubles itself in 10 years. When will it become fourfold of itself at the same rate of interest ?
Options:
A . 40 years
B . 15 years
C . 24 years
D . 20 years
Answer: Option D Answer: (d)Let the sum be x which becomes 2x in 10 years.Hence, 4x in 20 yearsMethod 2 : Unitary Method can also be used.Using Rule 5,Here, m = 2, t = 10Time taken to become 4 times = $2^2$ times= t × n = 10 × 2 = 20 years
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