If 1 out of 5 is a vowel, then 4 out of 5 are consonants. Now you have to set your 1 out of 5 equivalent to something out of 20 (your total number of cards) so 4 out of 20 will be vowels and 16 of 20 are consonants. (because 4 + 16 = 20) :)

1/5 x 20 = 4 vowels

20 - 4 = 16 consonants