Find rate from true discount: The true discount on a sum of ₹ 50,000 for 4 years is ₹ 2,000 at a certain simple-interest rate per annum. Find the annual rate of interest.

Difficulty: Medium

Correct Answer: 1%

Explanation:


Introduction / Context:
True discount TD for a future sum S due after t years at rate r (per annum) under simple interest is TD = S * r * t / (1 + r * t). Knowing TD, S, and t allows solving for r. This is a standard algebraic inversion.


Given Data / Assumptions:

  • S (future sum) = ₹ 50,000.
  • t = 4 years.
  • TD = ₹ 2,000.


Concept / Approach:
Let u = r * t. Then TD = S * u / (1 + u). Solve u from 2,000 = 50,000 * u / (1 + u), then compute r = u / t.


Step-by-Step Solution:

2,000(1 + u) = 50,000u ⇒ 2,000 = 48,000u ⇒ u = 2,000 / 48,000 = 1/24 ≈ 0.0416667.r = u / t = (1/24) / 4 = 1/96 ≈ 0.0104167 ≈ 1.04%.Closest standard rate in the options = 1%.


Verification / Alternative check:
Using r ≈ 1.0417% gives TD ≈ 50,000 * (0.010417*4) / (1 + 0.010417*4) ≈ 2,000; rounding to the nearest listed rate is appropriate.


Why Other Options Are Wrong:
1.5%, 2%, and 5% produce TD far from ₹ 2,000 for 4 years; 0.75% underestimates.


Common Pitfalls:
Omitting the denominator (1 + r t) and equating TD to S r t, which would overstate the rate.


Final Answer:
1%

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