Why does my child think 1/2 + 1/3 = 2/5?
Your child adds the tops and the bottoms of fractions, such as 1/2 + 1/3 = 2/5.
Why children do it
Adding the tops and adding the bottoms looks like the natural way to add fractions, and it's exactly how fractions are multiplied, which makes it more confusing. But the bottom number is the size of the pieces, and halves and thirds are different sizes. They only add once they're cut into pieces of the same size: 1/2 + 1/3 is 3/6 + 2/6, which is 5/6.
How to help at home
Draw both fractions as parts of the same-sized bar: they can only be added once the parts are the same size.
The right way
Calculate: 1/2 + 1/5
- 5/10 + 2/10Make both bottoms 10: 2 and 5 both go into 10.
- 7/10Add the tops. The bottom stays 10.
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