How to see it
You can only add things that are the same size. Thirds and quarters are not. So cut both into twelfths. Find a common denominator, the lowest number both bottoms go into. Then you are adding twelfths to twelfths, and only the tops change.
A worked example
Two thirds is eight twelfths. One quarter is three twelfths. Eight plus three is eleven. Eleven twelfths. For mixed numbers, convert to improper fractions first or do wholes and fractions separately.
The common mistake: adding the denominators
The trap is adding the tops and the bottoms: 2 plus 1 over 3 plus 4, three sevenths. That is smaller than two thirds, and adding cannot make a number smaller.
Try one
Five sixths take away one quarter.
- A4/2
- B7/12
- C4/10
- D1/2
Show the answerHide the answer
Right: B, 7/12
10/12 − 3/12.