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

Sentence: 'a young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'
5

Sentence: 'a bunch of broken down trains sit in a yard'. Find frequency of the letter 'n'
5

Sentence: 'an image of a female tennis player hitting her racket'. Find frequency of the letter 'k'

1