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

Sentence: 'a couple of girls are hanging out at a restaurant'. Find frequency of the letter 'l'
2

Sentence: 'a view from a bathroom shows an open door'. Find frequency of the letter 'o'
7

Sentence: 'a group of men standing behind a large flag banner'. Find frequency of the letter 'e'
4