Simple Interest – Find the annual rate from final amount: Mr. Deepak invests ₹ 21,250 for 6 years at simple interest. The total amount received at the end of 6 years is ₹ 26,350. At what annual simple-interest rate (per annum) did his money grow?

Introduction / Context:
Simple interest problems often ask for one missing variable among principal, rate, time, or amount. Here, the principal (₹ 21,250), time (6 years), and final amount (₹ 26,350) are given; we must determine the annual simple-interest rate. Simple interest grows linearly with time, making back-calculations straightforward when any three of the four quantities are known.

Given Data / Assumptions:

• Principal P = ₹ 21,250
• Time t = 6 years
• Amount A = ₹ 26,350
• Simple interest I = A − P
• Rate r is the annual rate in percent per annum

Concept / Approach:
For simple interest, I = P * r * t / 100 and A = P + I. When P, t, and A are known, compute I and solve for r using r = (I * 100) / (P * t).

Step-by-Step Solution:
I = A − P = 26,350 − 21,250 = 5,100r = (I * 100) / (P * t)r = (5,100 * 100) / (21,250 * 6)r = 510,000 / 127,500 = 4Therefore r = 4% per annum

Verification / Alternative check:
Check I using r = 4%: I = 21,250 * 4 * 6 / 100 = 21,250 * 0.24 = 5,100A = P + I = 21,250 + 5,100 = 26,350 (matches the given amount)

Why Other Options Are Wrong:
5%, 6%, 8%, and 12% would produce higher interest than ₹ 5,100 over 6 years for the same principal, so they do not match the stated final amount.

Common Pitfalls:
Confusing amount with interest, or using compound interest formulas when the question clearly states simple interest, often leads to incorrect rates. Always compute interest first, then substitute into the simple interest formula.

Final Answer:
4% per annum