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

Sentence: 'a silver train crossing the road on tracks'. Find frequency of the letter 'r'
5

Sentence: 'a man with a skateboard that is in the street'. Find frequency of the letter 'o'
1

Sentence: 'a laptop computer sitting on top of a table'. Find frequency of the letter 'a'

4