Percentages & Simple Interest
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Lesson overview
Percentages, profit and simple interest problems.
Go to lesson quiz →Exam guidance · WAEC WASSCE Paper 1 + Paper 2 Lens
Pacing
Objective speed on Paper 1 + structured step-by-step method marks on Paper 2 Theory/Practical.
How WAEC marks this
Examiners award method marks (M1) for stating the formula/definition and accuracy marks (A1) ONLY when SI units are written.
Watch out for
Writing bare numbers without working or omitting SI units loses up to 50% of Paper 2 marks even when the final value is right.
Lesson content
Overview — Percentages & Simple Interest
Percentages, profit and simple interest problems. This lesson is at the beginner level — by the end, you'll be able to recognise this question type, apply a reliable method, and avoid the traps that cost marks.
Key Concept
A percentage is a fraction out of 100. To find what percentage one number is of another: (part ÷ whole) × 100. Simple interest is calculated only on the original amount: Interest = Principal × Rate × Time ÷ 100.
Why It's Tested This Way
Percentage and interest calculations are tested because they map directly onto real-life money situations (discounts, profit, loans) — WAEC wants candidates who can apply maths outside the classroom, not just manipulate abstract numbers.
Method
For percentage increase/decrease problems, find the actual change first, then divide by the ORIGINAL value (not the new value) before multiplying by 100.
Worked Example
Worked example: A price rises from ₦200 to ₦250. Increase = ₦50; percentage increase = 50 ÷ 200 × 100 = 25%.
Second Worked Example
Second worked example: A price drops from ₦500 to ₦400. Decrease = ₦100; percentage decrease = 100 ÷ 500 × 100 = 20%.
Common Traps
Common trap: dividing the change by the NEW value instead of the original when computing percentage increase or decrease — this always gives the wrong answer.
Lesson Summary
Remember: (1) identify what is being asked, (2) apply the WAEC strategy, (3) check against the traps. Complete the 10-question quiz below to unlock the next lesson — you need at least 60% to pass.
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