Ratio and rate · Usually met around P6
What a ratio is
Terms in the words' order; the whole is the parts added.
How to do it
Method
Terms in the words' order; the whole is the parts added
3 red, 5 blue: red : blue = 3 : 5, red : whole = 3 : 8
Another way
Bar model
▭▭▭ / ▭▭▭▭▭ → 3 : 5
In full
To compare how many parts two amounts have, use a ratio: with 3 red beads and 5 blue ones, the ratio of red to blue is 3 : 5, said “3 to 5”. It means that for every 3 parts red there are 5 parts blue. Write the terms in the order the words name them: “blue to red” is 5 : 3. A ratio can compare a part with a part (red : blue = 3 : 5) or a part with the whole: there are 3 + 5 = 8 parts in all, so red : whole = 3 : 8. Or draw a bar model: a box a part, 3 boxes for red and 5 for blue, and you can see at a glance how many boxes each part and the whole have.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
In the picture, the ratio of 🎈 to all the things is □ : □.
Fill in each box
Show the answer and working
Answer: 7，11
There are 7 + 4 = 11 things in all, and 7 🎈: 7 : 11.
Example 2
The ratio of red beads to blue beads is 8 : 5. The ratio of red beads to all the beads is 8 : □.
Show the answer and working
Answer: 13
Red : blue = 8 : 5, so all the beads are 8 + 5 = 13 parts: 8 : 13.
Example 3
Chi-keung has 8 storybooks and 5 comics. The ratio of comics to storybooks is 5 : 13.
True or false?
Show the answer and working
Answer: False
Comics come first: 5 comics to 8 storybooks is 5 : 8.
Practise it in the game →Ratio and rate · Free to start, unlimited questions, no download