The probability is given by :
Note that there are 5 vowels in the English Alphabet with 26 total letters and repetition is allowed.
So the event of selecting all vowels is :
Event = 5 x 5 x 5 x 5 x 5 = 5^5
Total Events = 26 x 26 x 26 x 26 x 26 = 26^5
The probability will be :