Question

A certain sum doubles in 7 years at simple interest. In how many years the same sum under the same interest rate will become 4 times of itself.
1. 10 years
2. 14 years
3. 21 years
4. 28 years

Answer 1

Correct Answer - Option 3 : 21 years

Given:

Time (T₁) = 7 years

Time (T₂) = ?

Formula used:

Simple interest = Amount – Principal

Simple Interest (I) = (P × R × T)/100

Where, 

P → Principal

R →  Rate 

T → Time

Calculations:

Case I

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.