What is proportion?
Proportion means two ratios that are equal. For example, 1 : 2 is the same as 2 : 4, because both compare in the same way. We can grow or shrink a ratio by multiplying or dividing both numbers by the same amount.
Key idea
If amounts stay in proportion, then when one amount is multiplied, the other is multiplied by the same number.
Using proportion
Proportion is useful for shopping and cooking. A handy trick is to first find the value of one item, then multiply to find the value of many. This is called the unitary method.
Worked example
1 pencil costs RM2. How much do 3 pencils cost? The number of pencils is multiplied by 3 (1 → 3), so the cost is also multiplied by 3: RM2 × 3 = RM6. So 3 pencils cost RM6. Going the other way, if 4 pencils cost RM8, then 1 pencil costs RM8 ÷ 4 = RM2.
Remember
- Equal ratios stay in proportion.
- To find the cost of 1, divide.
- To find the cost of many, multiply.