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

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

Sentence: 'a double decker transit bus parks by a blue on the side of the street in front of a brick building in the city'. Find frequency of the letter 't'
9

Sentence: 'a platform of various fruits that have different shapes and sizes'. Find frequency of the letter 'r'
4