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

Sentence: 'a cat that is standing next to an open suitcase'. Find frequency of the letter 'e'
3

Sentence: 'three birds are sitting on a tree in a forest'. Find frequency of the letter 'h'
1

Sentence: 'the police station must be in one of these large brick buildings'. Find frequency of the letter 's'

4