A sum of money at simple interest doubles in 7 years. It will become four times in:

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Allahabad High Court Group D (Mains) Exam Official Paper (Held On: 02 Apr, 2023)
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  1. 42 years
  2. 18 years
  3. 21 years
  4. 38 years

Answer (Detailed Solution Below)

Option 3 : 21 years
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Detailed Solution

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Given:

A sum of money doubles in 7 years at simple interest.

Formula Used:

Simple Interest (SI) = Principal (P) × Rate (R) × Time (T) / 100

Amount (A) = Principal (P) + Simple Interest (SI)

Calculation:

Let the principal amount be P.

Since the amount doubles in 7 years, the simple interest earned in 7 years is P.

SI in 7 years = P

Using the formula for SI:

P = P × R × 7 / 100

⇒ 100 = R × 7

⇒ R = 100 / 7

To find the time taken for the amount to become four times (4P):

Simple Interest needed = 4P - P = 3P

Using the formula for SI:

3P = P × R × T / 100

⇒ 3P = P × (100 / 7) × T / 100

⇒ 3 = T / 7

⇒ T = 3 × 7

⇒ T = 21 years

The amount will become four times in 21 years.

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