A certain sum doubles in 7 years at simple interest. The same sum under the same interest rate will become 4 times in how many years

Explanation:

Let principal be x

So, Amount after 7 years = 2x

Simple interest = Amount – Principal

⇒ 2x – x = x

Interest = Principal

so x = (x × R × 7)/100

⇒ R = 100/7 %

Case II

Amount = 4x

Interest = 4x – x = 3x

Interest = 3 × Principal

so 3x = [x × (100/7) × T2]/100

⇒ T2 = 21 years

∴ The correct answer is 21 years.