Question : A sum amount doubles in 8 years by simple interest. Then the rate of simple interest per annum is:
Option 1: 10%
Option 2: 12.5%
Option 3: 15%
Option 4: 20%
Correct Answer: 12.5%
Solution :
Let the sum be $P$.
Time, $n$ = 8 years
Total amount for 8 years = $2P$
Simple Interest = total amount – sum = $2P – P$ = $P$
Simple Interest after $n$ years = $\frac{PnR}{100}$ where $P$ is principal and $R$ is the rate of interest per annum.
$P$ = $\frac{P×8×R}{100}$
⇒ $R$ = $\frac{100}{8}$ = 12.5%
Hence, the correct answer is 12.5%.
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