Ok. Shygrneyzs' answer is essentially correct but the explanation is poor. Let's get our terminology correct. If we answer questions using the wrong terms we'll only make the original poster more confused in the future.
85% is short hand for 85 percent or 85 per cent which really means 85 for every hundred (cent is essentially Latin for 100). So if you have 85 of one thing for every 100 of another you've got the fraction 85/100.
To reduce 85/100 you must find the greatest common factor (GCF) of both 85 and 100 and divide both numbers by that GCF. Common denominator is used when adding or subtracting fractions, not when reducing a fraction (it doesn't even mean anything with regards to a single fraction).