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 piece of meat covered in ketchup next to a hamburger on a green plate'. Find frequency of the letter 'g'
2

Sentence: 'a blue airplane in a blue, cloudless sky'. Find frequency of the letter 'l'

5