In this task, you need to count the number of times the given letter appears in the given sentence.

Sentence: 'a white police vehicle parked on a paved street'. Find frequency of the letter 'a'
4

Sentence: 'a black computer with a screen reading "press any key to begin."'. Find frequency of the letter 'a'
5

Sentence: 'a guy on skis is being pulled by a boat full of people'. Find frequency of the letter 'l'

5