Why does my child think a 2:3 ratio with 12 boys means 13 girls?
Your child adds instead of multiplying to scale a ratio, such as 2:3 with 12 boys giving 13 girls instead of 18.
Why children do it
Going from 2 to 12 is "add 10" to a child who thinks in adding, so they add 10 to the other part too. But a ratio keeps its proportions only when both parts are multiplied by the same number: 2 was multiplied by 6, so 3 becomes 18.
How to help at home
Ask what the first part was multiplied by. Ratios grow by multiplying both parts, not adding.
The right way
The ratio of boys to girls in a class is 2:3. There are 8 boys. How many girls are there?
- 8 ÷ 2 = 4The boys' part went from 2 to 8: that's 4 times as many.
- 3 × 4 = 12So multiply the girls' part by 4 as well, rather than adding.
- 12There are 12 girls.
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