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

Sentence: 'a group of people standing on grass with a train in the background'. Find frequency of the letter 'n'
6

Sentence: 'a green bus driving in a street filled with heavy traffic'. Find frequency of the letter 'v'
2

Sentence: 'elephants in a field with trees in the foreground'. Find frequency of the letter 'p'

1