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

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

Sentence: 'a red and white train passing a train station'. Find frequency of the letter 'a'
7

Sentence: 'two birds eating a fruit on a tree'. Find frequency of the letter 'n'

2