| What you do | Worked example |
|---|---|
| Add the numbers of the ratio to find how many parts there are. | 3 + 5 = 8 parts |
| Divide the total by the number of parts. | 800 / 8 = 100 Birr for one part |
| Multiply each number by the value of one part. | 3 x 100 = 300 Birr and 5 x 100 = 500 Birr |
| Add the shares back to check. | 300 + 500 = 800, the total you started with |
| When the question gives one person's share instead of the total, divide that share by its own number. | A receives 300 Birr for the 3, so one part is 100 Birr and B receives 500 Birr |
| Simplify a ratio by dividing every number in it by their highest common factor. | HCF(12, 18) = 6, so 12 : 18 = 2 : 3 |
| Put both quantities in the same unit before you write the ratio. | 50 cm : 2 m becomes 50 : 200, which is 1 : 4 |
When you use it
Use when a question hands you a total and a ratio and asks for each share, such as splitting 800 Birr between two people in the ratio 3 : 5. Add the numbers of the ratio to get the parts, divide the total by that number to get one part, then multiply each number by one part.
Watch out
Dividing by the wrong number is what costs the marks. Sharing 800 Birr in the ratio 3 : 5, students divide by 2 because there are two people, or by 3 because 3 is the first number. There are 8 parts, so one part is 100 Birr and the shares are 300 and 500. When the question gives one person's share instead of the total, divide by that person's own number: A's 300 Birr stands on the 3, so divide by 3 and not by 8. Add the shares back every time. If they do not give the total, the wrong number was used.