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 man that is on his cell phone'. Find frequency of the letter 's'
2

Sentence: 'a little boy trying to open a fire hydrant'. Find frequency of the letter 'o'
3