Question
A certain sum of money becomes 4 times of itself in 15 years at simple interest. In how many years does it become double of itself at the same rate of simple interest?
Solution
Let the principal = Rs. x, Amount = Rs. 4x, Time = 15 years ∴ Simple Interest = Rs. (4x – x) = Rs. 3x Rate = [(3x × 100)/(x × 15)]% p.a. = 20% p.a. Now, Principal = Rs. x, Amount = Rs.2x, Rate = 20%p.a. Simple Interest = Rs. (2x − x) = Rs. x ∴  Time = (x × 100)/(x × 20) = 5 years.
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