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

Sentence: 'an image of a cautionary construction sign'. Find frequency of the letter 'a'
5

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

Sentence: 'a wet polar bear in a zoo habitat'. Find frequency of the letter 'e'
2