Question : A certain sum doubles in 7 years at Simple Interest. The same sum under the same interest will become 4 times in how many years?
Option 1: 14 years
Option 2: 28 years
Option 3: 21 years
Option 4: 10 years
Correct Answer: 21 years
Solution :
Let the sum be $P$.
Total amount for 7 years = $2P$
Simple Interest for 7 years = $2P – P$ = $P$
If total amount = $4P$,
Simple Interest = $4P-P$ = $3P$
Since in 7 years interest is $P$ then $3P$ interest is received in 21 years.
Hence, the correct answer is 21 years.
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