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

Sentence: 'a baseball player dropping his bat and starting to run'. Find frequency of the letter 'o'
2

Sentence: 'a train is sitting at a train station'. Find frequency of the letter 'r'
2

Sentence: 'a couple walking their dog on a dirty street'. Find frequency of the letter 'e'

4