We know that we have five vowels in the alphabet. Since the deck has uppercase and lower case cards this means that it has a total of 10 vowels.
The probability is given by the quotient between the number of favorable outcomes and the number of total outcomes; then in this case we have:
![P=(10)/(52)=(5)/(26)](https://img.qammunity.org/2023/formulas/mathematics/college/p5ucdpzfuyqlrqwhrczoq93jsnzswbzrml.png)
Therefore the probability of selecting a vowel is 5/26