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

Sentence: 'a train going over a bridge with some buildings in the background'. Find frequency of the letter 'd'
3

Sentence: 'a group of men standing on the side of a boat'. Find frequency of the letter 'o'
5

Sentence: 'students are taking notes for class on their laptops'. Find frequency of the letter 'n'

4