1) That contain at least one vowel, if letters can be repeated? $26^8-21^8$
2) That contain exactly one vowel, if letters can be repeated? $8\cdot 5\cdot 21^7$
3) That start with an X and contain at least one vowel, if letters can be repeated? $1\cdot 26^7-1\cdot 21^7$
Assume only upper-cased letters are used.
I'm just trying to intuitively understand what's going on here. Can anyone explain in a clear and concise manner?
Thank you!