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

Sentence: 'a double-decker bus covered in children's illustrations driving next to a van'. Find frequency of the letter 'a'
4

Sentence: 'there is a person standing on a blue towel'. Find frequency of the letter 'n'
4

Sentence: 'a baseball player goes in for a side catch as an apposing player slides into base'. Find frequency of the letter 'a'

11