Lesson 1 of 4 · Grade 8 Mathematics
Ratio and proportion
15 min
Ratio
A ratio compares two quantities of the same kind: 12 boys and 18 girls → 12 : 18 = 2 : 3 (divide by 6).
Sharing in a ratio
Share KES 1,500 between Akinyi and Baraka in the ratio 2 : 3.
- Total parts = 2 + 3 = 5
- One part = 1,500 ÷ 5 = 300
- Akinyi: 2 x 300 = KES 600; Baraka: 3 x 300 = KES 900
Direct proportion
As one quantity increases, the other increases at the same rate.
If 4 books cost KES 600, then 1 book costs 150 and 7 books cost KES 1,050.
Inverse proportion
As one quantity increases, the other decreases.
If 6 workers take 10 days to build a wall, 12 workers take 6 x 10 ÷ 12 = 5 days.
Key points to remember
- Simplify ratios like fractions
- Share: total parts → one part → each share
- Direct: both increase together
- Inverse: one increases, the other decreases
Test yourself
