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

Sentence: 'some sheep are standing in a grassy field'. Find frequency of the letter 'n'
3

Sentence: 'a gray airplane with metal petals on the wings takes off from an airport'. Find frequency of the letter 'i'
4

Sentence: 'a station with people waiting for the train'. Find frequency of the letter 'l'

1