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

Sentence: 'a small white plane sitting on top of the ocean'. Find frequency of the letter 'e'
4

Sentence: 'a couple of buildings that has a bus out front'. Find frequency of the letter 'a'
4

Sentence: 'a blue subway train in the middle of a city'. Find frequency of the letter 'b'

2