Question

how do you add fractions with uncommon denominators?

Answer 1

You need to change the fractions to have a common denominator.
Check this site to see how to change fractions so they have a common denominator.

http://www.themathpage.com/ARITH/add-fractions-subtract-fractions-1.htm

Answer 2

You change the fractions to a common denominator, then add and reduce the fraction ( if possible)
example: 1/3 + 1/4 = ?
4/12 + 3/12 = 7/12

This can not be further reduced.