Two quantities are in direct proportion when they increase or decrease in the same ratio. For example, you could increase something by doubling it, or decrease it by halving.
Here is a typical problem:
Twelve pencils cost . Find the cost of pencils.
To solve this problem, we need to know the cost of one pencil.
We know that pencils cost , so if we divide by to give us the cost of one pencil:
So pencil costs . Now we need to know the cost of pencils. We multiply by .
So pencils cost .
If you had a problem working out the answer, the basic method to remember is to divide by how many you know, then multiply by what you want to know.
Now try this one:
Jenny buys felt-tip pens. It costs her . How much would pens have cost?
The answer is .
You divide by , then multiply the answer by .