Linking simple interest and true discount: On the same sum and time, the simple interest is ₹24 while the true discount is ₹22. Find the sum due (face value) on which these were computed.

Difficulty: Medium

Correct Answer: ₹264

Explanation:


Introduction / Context:
This classic TD–SI linkage uses the time-rate factor R = r * t (in decimal). For the same “sum due” S and same time, the simple interest on S is S * R, while the true discount is S * R / (1 + R). Their ratio eliminates S and gives R directly; then recover S from the given SI value.


Given Data / Assumptions:

  • SI on S for time t at rate r: SI = ₹24.
  • True discount on the same S, time, rate: TD = ₹22.
  • Let R = r * t (in decimal units).


Concept / Approach:
For the same S and R: SI = S * R and TD = S * R / (1 + R). Hence TD/SI = 1/(1 + R). Use the numerical values to find R. Then S = SI / R.


Step-by-Step Solution:

TD / SI = 22 / 24 = 11 / 12 = 1 / (1 + R).⇒ 1 + R = 12 / 11 ⇒ R = 1 / 11.SI = S * R ⇒ 24 = S * (1/11) ⇒ S = 24 * 11 = ₹264.


Verification / Alternative check:
TD predicted = S * R / (1 + R) = 264 * (1/11) / (12/11) = 24 / 12 = ₹22, matching the given TD.


Why Other Options Are Wrong:
₹220, ₹288, ₹295, ₹242 do not satisfy both SI = 24 and TD = 22 under a single R.


Common Pitfalls:
Assuming SI equals TD (that holds when SI is on present worth, not on the sum due S). Here SI is on S.


Final Answer:
₹264

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