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

Sentence: 'a passenger train that is going over a bridge'. Find frequency of the letter 't'
3

Sentence: 'people playing wii bowling in a modern library'. Find frequency of the letter 'o'
3

Sentence: 'a person skateboarding on a concrete ramp and a young child standing at the top of the ramp with a skateboard'. Find frequency of the letter 'a'

13