Direct proportion involves situations where two values vary, but the ratio between the values stays the same.
You can use direct proportion to answer questions like the one below:
Starting fact: 10 calculators cost £120
To solve this, we need to change "10 calculators" into "8 calculators".
We start by finding out what 1 calculator costs, by dividing both parts of our fact by 10:
New fact: 1 calculator costs £12
Now we need to change "1 calculator" into "8 calculators".
We can do this by multiplying both parts of our fact by 8:
Answer: 8 calculators cost £96
Method
Every time we want to answer a question like this, we start by altering the fact so that it just shows what one of
something costs. In the above example, we worked out what one calculator costed. We could then use this to
work out what eight calculators costs. This is sometimes called the unitary method.
Practice Questions
Work out the answer to each of these questions then click on the button marked
to see whether you are correct.
(a) If 5 bags of sweets contain 90 sweets in total, calculate how many sweets seven bags will contain?
(b) If 25 floppy disks cost £5.50, calculate the cost of 11 floppy disks.
