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

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

Sentence: 'a man standing on a surf board in the ocean'. Find frequency of the letter 'f'
1

Sentence: 'two bright green passenger trains pass by with people standing in the doorways'. Find frequency of the letter 'a'

5