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How to Solve WAEC Simple Interest Questions, Step by Step

Simple interest questions show up almost every year in WAEC and NECO maths, and they follow the same pattern once you know the formula. Here's exactly how to handle them.

The formula

Simple Interest (I) = (P × R × T) / 100, where P is the principal (the amount deposited or borrowed), R is the rate per annum as a percentage, and T is the time in years.

Worked example

Question: Bola deposits ₦5,000.00 in a savings account. If she is paid an interest rate of 5% per annum for 2 years, how much is the simple interest?
  1. Write down what you know: P = 5,000, R = 5, T = 2.
  2. Put the numbers into the formula: I = (5,000 × 5 × 2) / 100.
  3. Multiply the top: 5,000 × 5 = 25,000, then 25,000 × 2 = 50,000.
  4. Divide by 100: 50,000 / 100 = 500.
Answer: ₦500.00

Common mistake to avoid

Many students forget that the question sometimes asks for the total amount (principal plus interest), not just the interest itself. Always re-read the question after solving. If it says "how much is the interest," you stop at I. If it says "how much will she have altogether," you add P + I.

Exam tip: In a multiple-choice WAEC question, if your answer doesn't match any option, check whether the options are testing the interest alone versus the total amount, that's the most common reason for a mismatch.
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