Equivalent ratios describe the same relationship, like 3 : 5 and 6 : 10. To find a missing quantity, work out what one unit is worth first, then scale up. For example, in 3 : 5 = 12 : ?, the 3 became 12 — that is times 4 — so multiply 5 by 4 as well to get 20. Method: find one unit, then multiply.
Worked example
Problem Find the missing number in the equivalent ratios 3 : 5 = 12 : ▢.
Compare the matching parts: 3 became 12, so the scale factor is 12 ÷ 3 = 4.
Multiply the other part by the same factor: 5 × 4 = 20.
So 3 : 5 = 12 : 20.
Tip Equivalent ratios scale every part by the same number.