The english alphabet contains $21$ consonants and $5$ vowels. How many strings of $6$ lowercase letters of the English alphabet contain
- a) exactly 1 vowel
- b) exactly 2 vowels
- c) at least one vowel
- d) at least two vowels
The english alphabet contains $21$ consonants and $5$ vowels. How many strings of $6$ lowercase letters of the English alphabet contain