Question:

At what rate of interest per annum compounded annually will a sum of money become 81 times in 4 years?

Updated On: Mar 4, 2025
  • 50%
  • 100%
  • 150%
  • 200%
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The Correct Option is D

Solution and Explanation

Calculating the Rate of Interest Using Compound Interest Formula

Formula for Compound Interest: 

A = P (1 + r / 100)ᵗ

Given Information:

  • Final Amount (A) = 81P
  • Principal (P)
  • Time (t) = 4 years

Step-by-Step Calculation:

81P = P (1 + r / 100)⁴

Canceling P from both sides:

81 = (1 + r / 100)⁴

Taking the fourth root:

∜81 = 1 + r / 100

3 = 1 + r / 100

Solving for r:

r / 100 = 2

r = 200%

Final Answer:

Thus, the correct answer is 200% (Option D).

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