Skill: Find a missing value in an equivalent ratio

An equivalent ratio keeps the same multiplicative relationship between the two quantities. To find a missing value, find the factor that changes one known term into its matching term, then use that same factor on the other term.

Example: Find $x$.

$$3:5 = 12:x$$

1. Compare the first terms: $3$ becomes $12$.

$$3 \times 4 = 12$$

So, the scale factor is $4$.

2. Multiply the other term by the same factor:

$$5 \times 4 = 20$$

3. Therefore,

$$x=20$$

So the equivalent ratios are

$$3:5 = 12:20$$

Check: Both terms in $3:5$ were multiplied by $4$. Adding the same number would not work because equivalent ratios are made by multiplying or dividing both terms by the same factor.