ok, so there are 26 letters in the entire alphabet. What you have to do is set up a fraction pretty much.
so that is 6 vowels
and 26 letters...
your equation should be:
6/26
then you should technically reduce.
6/26 ->(รท2)->3/13
and that would be all you can do:)
so your overall answer would be;
3/13 (3 over 13)