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

Sentence: 'a little dog on a table while a dog groomer cuts its fur'. Find frequency of the letter 'r'
3

Sentence: 'a guy surfs a wave with his surfboard'. Find frequency of the letter 's'
4

Sentence: 'four jets are flying in formation across a blue sky'. Find frequency of the letter 'n'

3