How to Solve WAEC Simple Interest Questions, Step by Step

Simple interest questions appear almost every year in WAEC, NECO, and JAMB maths, and they follow a small, predictable set of patterns once you understand the formula and what each variable means. This guide walks through the formula itself, three full worked examples covering the different ways the question can be asked, the most common mistake students make, and a short FAQ for the trickier variations.

What is simple interest?

Simple interest is the extra money paid on a loan or earned on a savings deposit, calculated only on the original amount (the principal), not on any interest that has already accumulated. This is different from compound interest, where interest is calculated on the principal plus any interest already earned. WAEC and NECO almost always test simple interest at this level, since compound interest is usually a separate topic tested on its own.

The formula

Simple Interest (I) = (P × R × T) / 100

Where:

• P is the principal, the original amount deposited or borrowed
• R is the rate per annum, written as a plain number, not a decimal (5% becomes R = 5, not R = 0.05)
• T is the time, always converted to years before you calculate

Worked example 1: finding the interest

Worked example 2: finding the total amount

Sometimes the question doesn't stop at the interest, it asks how much the person will have in total. This is where many students lose easy marks.

Worked example 3: working backwards to find the rate

WAEC also likes to flip the question around, giving you the interest and asking you to find the rate or the time instead. The same formula still works, you just rearrange it.

The most common mistake

Many students forget that the question sometimes asks for the total amount (principal plus interest), not just the interest itself, as shown in worked example 2 above. 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" or "what is the total amount," you must add P + I. In a multiple-choice WAEC question, if your answer doesn't match any option, this mismatch between interest and total amount is the single most common reason.

Frequently asked questions

What if the time is given in months instead of years?
Convert it first. Divide the number of months by 12 to get years. For example, 6 months becomes T = 6/12 = 0.5.

What if the rate is given as a fraction, like 2½%?
Convert the mixed number to a decimal before using it in the formula. 2½% becomes R = 2.5.

Does the formula change for borrowing versus saving?
No. The formula is exactly the same whether the principal is a deposit earning interest or a loan accruing interest. Only the context of the question changes, not the maths.