Simple Interest – Find original principal from amount: At 5% per annum simple interest, a sum of money amounts to Rs 900 in 4 years. Find the original principal (sum).

Introduction / Context:
Given the final amount under simple interest, we can recover the principal using the linear growth model A = P * (1 + r * t).

Given Data / Assumptions:

• A = Rs 900
• r = 5% per annum
• t = 4 years

Concept / Approach:
Under SI, A = P * (1 + r * t). Rearranging, P = A / (1 + r * t). Use r = 0.05, t = 4.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• Rs 650 / Rs 550 / Rs 500 / Rs 800: do not satisfy A = 900 under 5% for 4 years.

Common Pitfalls:

• Mistaking compound interest for simple interest.
• Forgetting to convert percent to decimal.

Final Answer:
Rs 750.