Using ratios
Ratios can be used to solve many different problems - for example, with recipes, scale drawings or map work. A typical question will expect you to change a ratio - the reverse of cancelling down.
- Question
Sam does a scale drawing of his kitchen. He uses a scale of 1:100. He measures the length of the kitchen as 5.9m.
How long is the kitchen on the scale drawing? Give your answer in mm.
- Answer
The answer is 59mm.
You need to convert 5.9m to mm, then divided by 100 to give the answer.
If you did not get the correct answer, remember that the scale of 1:100 means that the real kitchen is 100 times bigger than the scale drawing.
5.9m = 590cm (multiplied by 100) = 5900mm (multiplied by 10)
So the scale drawing would be 5900 ÷ 100 = 59mm.
Another typical question will give you a recipe to modify for a different number of people.
- Question
A recipe to make lasagne for 6 people uses 300 grams of minced beef. How much minced beef would be needed to serve 8 people?
- Answer
The best way to approach a problem like this is to find out how much 1 person would need first.
- Six people need 300g.
- So 1 person needs 50g (300 ÷ 6).
- So 8 people need 50g × 8 = 400g.
Try this one.
- Question
A recipe for flapjacks requires 240g of oats. This makes 18 flapjacks.
What quantity of oats is needed to make 24 flapjacks?
- Answer
The answer is 320g.
You divide 240 by 18, then multiply the answer by 24.
If you had problems working out the answer, try to fill in the blanks here:
- Eighteen flapjacks need 240g.
- So 1 flapjack needs ?g (240 divided by 18).
- So 24 flapjacks need 13.3333 × ? = 320g.
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