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

Sentence: 'a person riding a skate board on a city street'. Find frequency of the letter 'a'
5

Sentence: 'a green bus driving in a street filled with heavy traffic'. Find frequency of the letter 'v'
2

Sentence: 'a baseball player wearing a white uniform and black socks waiting for the ball'. Find frequency of the letter 'l'

6