If a sum of money doubles itself on simple interest in 7 years, then in how many years will it become 4 times itself?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 21
Simple interest is charged only on the original sum, so it grows by the same amount every year.
The sum doubles in 7 years, so the interest for 7 years equals the sum itself, \(P\)
To become 4 times, the sum needs interest of \(4P-P=3P\)
\(3P\) takes three times as long as \(P\): \(3\times7=21\) years
Tip: Under simple interest, a sum that doubles in \(t\) years becomes \(n\) times in \((n-1)\,t\) years.