Filling a fraction to 1
You learn which fraction you need to add to make exactly 1 whole.
How to work it out
- A whole has the same number above and below the line: 1 is 88.
- Look how many parts you already have, and take that away from the denominator.
38
A worked example
38 + ? = 1
- One whole is 8 eighths: 1 = 88.
- You already have 3 eighths.
- Take the numerator away from the denominator: 8 − 3 = 5.
- You need 58 to make 1.
More examples
25 + ? = 1
- One whole is 55.
- Take 2 away from 5: 5 − 2 = 3.
- You need 35.
25
710 + ? = 1
- One whole is 1010.
- Take 7 away from 10: 10 − 7 = 3.
- You need 310.
710
Try it yourself
Which fraction do you need to add to make 1 whole?
712 + ? = 1
Which fraction do you need to add to make 1 whole?
15 + ? = 1
Watch out
You add the denominators together instead of just the numerators.
The denominator stays the same: 3 eighths plus 5 eighths is 8 eighths, not 8 sixteenths.
Tips
- Look at the denominator: that is the number of slices in 1 whole.
- Take the numerator away from the denominator to find how many slices are missing.
Why it works
A whole cake split into 8 slices is 88. If you have 3 slices, you need 5 more to make the whole cake. So you count on from the numerator to the denominator.