Using direct proportion

Understanding proportion can help in making all kinds of calculations. It helps you work out the value or amount of quantities either bigger or smaller than the one about which you have information. Here are some examples:

Example 1:
If you know the cost of 3 packets of batteries is £6.00, can you work out the cost of 5 packets?
To solve this problem we need to know the cost of 1 packet.
If three packets cost £6.00, then you divide £6.00 by 3 to find the price of 1 packet.
            (6 ÷ 3 = 2)
Now you know that they cost £2.00 each, to work out the cost of 5 packets you multiply £2.00 by 5.
            (2 x 5 = 10)
So, 5 packets of batteries cost £10.00

Example 2:
You've invited friends round for a pizza supper. You already have the toppings, so just need to make the pizza base. Looking in the recipe book you notice that the quantities given in the recipe are for 2 people and you need to cook for 5!

Pizza base - to serve 2 people:
          100 g flour    60 ml water
          4 g yeast    20 ml milk
          pinch of salt

The trick here is to divide all the amounts by 2 to give you the quantities for 1 serving. Then multiply the amounts by the number stated in the question, 5.

For 1 serving, divide by 2:
          100 g ÷ 2 = 50 g    60 ml ÷ 2 = 30 ml
          4 g ÷ 2 = 2 g    20 ml ÷ 2 = 10 ml

For 5 servings, multiply by 5:
          50 g x 5 = 250 g    30 ml x 5 = 150 ml
          2 g x 5 = 10 g    10 ml x 5 = 50 ml

The pinch of salt is up to you!