To add fractions the denominators must be the same. #3/9# can be simplified to #1/3#, so the sum becomes #1/3 + 4/5#. The lowest common denominator for #3# and #5# is #15#, so we multiply #1/3# by #5/5#, resulting in #5/15# and we multiply #4/5# by #3/3#, resulting in #12/15#. Now we can add the two fractions: