You can format the cell as Fraction, however in your example 2 1/2% = 1/40. So you will need a helper cell to change the value into a fraction. 2.5% is really .025. If we multiply by 100 we get 2.5 then format as Fraction we get 2 1/2

In your helper cell formatted as Fraction, enter:

=A1*100 (where A1 holds the percent value)

HTH

Bruce