How to insert fractions
To enter a fraction into a cell, leave a space between the whole number and the fraction.
For example, to enter , enter 6 7/8, and then press Enter.
When you select the cell, 6.875 appears in the Formula bar, and the cell entry appears as a fraction.
If you have a fraction only (for example, ), you must enter a zero first, like this: 0 1/8 - otherwise Excel will likely assume that you are entering a date. When you select the cell and look at the Formula bar, you see 0.125. In the cell, you see 1⁄8.
See also this tip in French: Comment insérer des fractions.