Hey all Im having trouble with this 1 problem, hope you guys can help me through it and understand it.

The English alphabet contains 21 consonants and five vowels. How many strings of six lowercase letters of the English alphabet contain:

a)exactly 1 vowel, the answer is 122523030, but I don't know how they got the answer. I did 21*21*21*21*21*5, but didn't get the same answer.

b)exactly two vowels, I did 21*21*21*21*5*5 but is wrong the answer is 72930375

c)at least one vowel: I don't quite understand the concept of calculating problems withat least or at most.d) at most 2 vowels

thanks for your help in advance