With fractions the whole number doesn't need to be divisible by the denominator. Fractions are just divisions. Say for example you had the fraction 25/15. Now obviously you can't divide 25 by 15 and that is why you would leave it as a fraction. What you can often do, and that is why I picked this example, is simplify fractions. What number goes into 15 and 25? - 5. So divide top and bottom by 5 to get 5/3. That will give you the fraction In its simplest form.
I'm not entirely sure that this is exactly what you meant, but your query was rather vague, so I hope this is helpful.
