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

Sentence: 'a railroad train pulling up to the train platform'. Find frequency of the letter 'o'
3

Sentence: 'three birds are sitting on a tree in a forest'. Find frequency of the letter 'h'
1

Sentence: 'a black bear sitting in the grass taking a rest'. Find frequency of the letter 'a'

6