Total Number of letters in the word ASSASSINATION, $n\left(S\right)$ = 13

Total number of Vowels in the word ASSASSINATION, $n\left(E\right)$ = 6 ( 3 A, 2 I, 1 O)

Probability for getting a vowel, $P\left(E\right)$ =$\dfrac{\text{n(E)}}{\text{n(S)}} = \dfrac{6}{13}$