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

Sentence: 'a person is riding in a cart pulled by a horse'. Find frequency of the letter 'a'
4

Sentence: 'a few giraffes standing in a field looking around'. Find frequency of the letter 'e'
3

Sentence: 'a zebra lying in the sand in the sun'. Find frequency of the letter 'n'
5