What you need to know
A ratio compares amounts of the same kind. Cordial mixed 1 : 4 means 1 part cordial for every 4 parts water. Order matters: 1 : 4 is not the same as 4 : 1. Ratios are simplified like fractions, by dividing every part by the same number, and the parts must be in the same units before you simplify. A rate compares two different kinds of quantity, such as kilometres per hour or dollars per kilogram.
Simplify 12 : 18 and 250 g : 1 kg
- 12 : 18. The highest common factor of 12 and 18 is 6. Divide both parts by 6: 2 : 3.
- 250 g : 1 kg. The units differ, so convert first: 1 kg = 1000 g, giving 250 : 1000.
- Divide both parts by 250: 1 : 4.
Equivalent ratios. Multiplying or dividing both parts by the same number gives an equivalent ratio: 2 : 3 = 4 : 6 = 10 : 15. This is how you scale a recipe. If 2 cups of flour go with 3 cups of milk, then 8 cups of flour (2 × 4) need 3 × 4 = 12 cups of milk. Check: 8 : 12 simplifies back to 2 : 3.
Dividing a quantity in a ratio. Add the parts of the ratio to find the total number of parts, divide the quantity by that total to find the value of one part, then multiply to find each share. Always check that the shares add back to the original amount.
Share $120 between Ava and Ben in the ratio 3 : 5
- Total parts: 3 + 5 = 8.
- One part: $120 ÷ 8 = $15.
- Ava gets 3 × $15 = $45. Ben gets 5 × $15 = $75.
- Check: $45 + $75 = $120.
A car travels 180 km in 2.5 hours. What is its average speed, and how far would it go in 4 hours at that speed?
- Speed is a rate: distance ÷ time. A unit rate tells you the amount per 1 unit, here per 1 hour.
- 180 ÷ 2.5 = 72 km/h. Check: 72 × 2.5 = 180.
- In 4 hours: 72 × 4 = 288 km.
Best buy: 500 g of rice for $2.40 or 2 kg of rice for $9.00?
- Compare the cost of the same amount. Use 100 g.
- 500 g for $2.40: $2.40 ÷ 5 = $0.48 per 100 g.
- 2 kg = 2000 g for $9.00: $9.00 ÷ 20 = $0.45 per 100 g.
- The 2 kg bag is the better buy, by 3 cents per 100 g.
| Ratio | First part as a fraction of the total | Second part as a fraction of the total |
|---|
| 1 : 1 | 12 | 12 |
| 1 : 3 | 14 | 34 |
| 2 : 3 | 25 | 35 |
| 3 : 7 | 310 | 710 |
Common mistake: turning the ratio 2 : 3 into the fraction 23. In a 2 : 3 ratio there are 5 parts altogether, so the first share is 25 of the total and the second is 35. The fraction 23 only tells you how the two parts compare with each other.
Common mistake: simplifying before the units match. 250 g : 1 kg is not 250 : 1. Convert to the same unit first (250 : 1000), then simplify to 1 : 4.
Check yourself
- Simplify 15 : 25.
- Simplify 40 cm : 2 m.
- Complete the equivalent ratio: 3 : 4 = ? : 20.
- Share 56 lollies between two friends in the ratio 2 : 5.
- A cyclist rides 250 km in 4 hours. What is her average speed?
- Which is the better buy: 6 apples for $4.20 or 10 apples for $6.50?
Answers: 1. 3 : 5 2. 1 : 5 (40 : 200) 3. 15 4. 16 and 40 (7 parts, so one part is 8) 5. 62.5 km/h 6. 10 apples for $6.50 (65 cents each, compared with 70 cents each)