I am having a hard time coming up with a way to calculate this.
There are 5 vowels so I think the number of strings that contain exactly 1 vowel would be:
$5$ × $8$ × $21^5$
But I am not sure where to go from here. I believe I need to find a number to subtract from this number.
Best Answer
There are
Thus there are $206453125$ admissible strings in total.