Last reviewed: 25 October 2026
Our Grade 8 mathematics revision guide covers the whole year. This page does something different. It starts from the questions Grade 8 learners actually get wrong on the maths CBC Challenge. The Challenge records which topic each missed question came from, without recording who answered it. In the early results, Grade 8 maths had a lower average score than the Grade 6 and Grade 9 maths quizzes. The most-missed topic, by a clear margin, was coordinates and graphs, followed by ratio, circles and money. It is still a small sample, so treat it as a pointer, not a verdict, but these are also topics where one small misunderstanding costs a lot of marks.
On this page
1. Coordinates and graphs
The single biggest error is reading coordinates the wrong way round. A point is written (x, y): across first, then up or down. A memory trick: “along the corridor, then up the stairs”. So (3, −2) is 3 to the right and 2 down. Other common slips:
- Using an uneven scale on the axes, so plotted lines bend when they should be straight.
- Forgetting that every point on the y-axis has x = 0, and every point on the x-axis has y = 0.
- For a straight-line graph such as y = 2x + 1, working out only one point. Make a small table with at least three values of x. If the three points do not line up, one calculation is wrong.
The Cartesian coordinates page on Maths Is Fun has an interactive grid that makes the order stick.
2. Ratio
To share an amount in a ratio, add the parts, find one part, then multiply. To share KES 600 in the ratio 2 : 3, there are 5 parts, so one part is KES 120 and the shares are KES 240 and KES 360. Check that the shares add back up to the total. Learners often divide by one of the ratio numbers instead of the total number of parts. Ratios must also use the same units: 50 cm : 2 m is 50 : 200, which simplifies to 1 : 4. See the Maths Is Fun ratio explainer and our ratio, proportion and rates revision.
3. Circles
Circumference = πd (or 2πr), and area = πr². The traps are using the diameter in the area formula, and forgetting that area is in square units. If a question gives a diameter of 14 cm, the radius is 7 cm, so the area is 22/7 × 7 × 7 = 154 cm². Ask your child to write “r = …” before every circle calculation.
4. Money
Grade 8 money questions include profit and loss, discount, commission and simple interest. Always identify the base. Percentage profit is calculated on the buying price. A trader who buys at KES 400 and sells at KES 500 makes 100 ÷ 400 × 100 = 25%, not 20%.
5. Probability, angles and algebra
These came next on the list. Quick reminders:
- Probability is always between 0 and 1. A probability of 1.2 means a mistake.
- Interior angles of a polygon add up to (n − 2) × 180°.
- Algebraic expressions: collect only like terms. 3a + 2b cannot be simplified to 5ab.
How to use this page
- Your child takes one Grade 8 maths Challenge set.
- Check the “Revise next” topics on the results screen against the sections above.
- Spend ten minutes on one section, then try a fresh set the next day.
Short, daily practice over the holiday will do more than a long session once a week. Our long holiday revision plan shows how to fit it in.