Idea 12of 22Builds onAre two ratios equivalent?
Find a missing term
How do you find the missing number in two equivalent ratios?
Today you'll discover
- which pair tells you the scale factor
- how to use it on the other term
- how to check by counting up
Think firstJuice to water is 3 : 5
Juice and water are mixed 3 : 5. How much water goes with 12 cups of juice?
Decide first, then press Next. Find the pair where both numbers are known.
A ratio table: cups of juice on top, cups of water below. 3 and 5 in the first column; the last column has 12 cups of juice and a question mark for the water. Curved arrows marked times 4 join them; then the last column is ringed with 12 and 20 and 3 : 5 = 12 : 20 under it. Then the whole table with steps of plus 3 and plus 5. The last step asks about girls and boys.
See the whole ideathe big idea and the key word, in one place
The big idea
Find the scale factor from the pair you know, then use it on the other term.
Working it out3 : 5 = 12 : ?. 3 × 4 = 12, so ? = 5 × 4 = 20.
Easy to get wrongDo not add the gap. From 3 to 12 is 9 more, but the water does not get 9 more; it gets 4 times as much.
Key word
- missing term
- the one number left blank in two equivalent ratios, found with the scale factor
Ready to use it?
- Find the missing term in two equivalent ratios.
- Check a missing term with a ratio table.