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Question

Sapna invested ₹20,300 on simple interest, partly at the rate of 8% per annum and partly at the rate of 6% per annum. If she earns equal interests from the two investments after 6 years, then find the sum invested at the rate of 8% per annum (in ₹).

This question was previously asked in
RRB NTPC 2024 CBT 1 Question Paper (28-Aug-2025) (Shift 3)
The correct answer is
8,700

Investment Breakdown

Let the total investment be $P_{total} = ₹20,300$.

The investment is split into two parts:

  • Sum 1 ($P_1$) invested at Rate 1 ($R_1 = 8\%$ per annum).
  • Sum 2 ($P_2$) invested at Rate 2 ($R_2 = 6\%$ per annum).

We know that $P_1 + P_2 = ₹20,300$.

The time period for both investments is $T = 6$ years.

Equal Simple Interest Calculation

The formula for Simple Interest (SI) is: $SI = \frac{P \times R \times T}{100}$.

Interest earned from the first investment ($SI_1$): $SI_1 = \frac{P_1 \times R_1 \times T}{100} = \frac{P_1 \times 8 \times 6}{100} = \frac{48 P_1}{100}$

Interest earned from the second investment ($SI_2$): $SI_2 = \frac{P_2 \times R_2 \times T}{100} = \frac{P_2 \times 6 \times 6}{100} = \frac{36 P_2}{100}$

The problem states that the interests earned are equal: $SI_1 = SI_2$.

Therefore, $\frac{48 P_1}{100} = \frac{36 P_2}{100}$.

Simplifying the equation: $48 P_1 = 36 P_2$.

Dividing both sides by 12: $4 P_1 = 3 P_2$.

This gives us a relationship between $P_1$ and $P_2$: $P_2 = \frac{4}{3} P_1$.

Finding the Sum Invested at 8%

Now substitute the expression for $P_2$ into the total investment equation ($P_1 + P_2 = 20300$):

$P_1 + \frac{4}{3} P_1 = 20300$

Combine the terms involving $P_1$: $\frac{3 P_1 + 4 P_1}{3} = 20300$ $\frac{7 P_1}{3} = 20300$

Solve for $P_1$: $7 P_1 = 20300 \times 3$ $P_1 = \frac{20300 \times 3}{7}$

Calculate the value: $P_1 = 2900 \times 3$ $P_1 = 8700$

The sum invested at the rate of 8% per annum is $₹8,700$.

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Similar Questions

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  2. Sapna invested ₹21,900 on simple interest, partly at 14% per annum and partly at 10% per annum. If she earns equal interests from the two investments after 2 years, then find the sum invested at 14% per annum (in ₹).
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Important Questions from Simple Interest

  1. Anil lent a sum of Rs. 5,000 on simple interest for 10 years in such a way that the rate of interest is 6% per annum for the first 2 years, 8% per anmum for the next 2 years and 10% per annum beyond 4 years. How much interest (in Rs.) will he earn at the end of 10 years?

  2. What will be the simple interest on a sum of Rs. 12000 at the rate of 15 percent per annum for three years ?

  3. If in 13 years fixed sum doubles at simple interest, what will be the interest rate per year? (correct to two decimal places)

  4. On simple interest a sum of Rs. 640 becomes Rs. 832 in 2 years. What will Rs. 860 become in 4 years at the same rate of simple interest?

  5. A certain sum amounts to Rs. 81840 in 3 years and to Rs. 92400 in 5 years at x% p.a. under simple interest. If the rate of interest is becomes (x + 2)%, then in how many years will the same sum double itself?

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