How to see it
In direct proportion, when one thing doubles, so does the other. The reliable method: find the cost of one, then scale up. Recipes, maps and prices all work this way. For maps, the scale tells you how many real centimetres each map centimetre stands for.
A worked example
One pen is £3.50 divided by 5, 70 pence. Eight pens is 8 times 70 pence, £5.60. For a recipe: 300 grams of flour for 4 people means 75 grams each, so 750 grams for 10.
The common mistake: adding instead of scaling
The trap is adding. From 5 to 8 pens is 3 more, so add 3 to the price? Prices do not work like that. The price grows by multiplying, not adding.
Try one
6 pencils cost £2.40. How much for 9?
- A£3.60
- B£2.43
- C£5.40
- D£3.00
Show the answerHide the answer
Right: A, £3.60
60. One pencil is 40p.