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

Sentence: 'two men sitting quietly on a park bench'. Find frequency of the letter 'a'
2

Sentence: 'a bus is driving on the street behind some cars'. Find frequency of the letter 'a'
2

Sentence: 'woman lunges forward as she's about to serve the tennis ball back to the other side'. Find frequency of the letter 'b'

3