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

Sentence: 'three men in military suits are sitting on a bench,'. Find frequency of the letter 'a'
3

Sentence: 'a couple of men sitting next to each other under an umbrella'. Find frequency of the letter 'n'
5

Sentence: 'president obama being presented with a large candle lit sheet cake in the oval office'. Find frequency of the letter 'r'

3