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

Sentence: 'a couple of buildings that has a bus out front'. Find frequency of the letter 'a'
4

Sentence: 'two people under an umbrella while walking down a sidewalk'. Find frequency of the letter 'i'
3

Sentence: 'two cows are looking into the camera in a heard of cattle that appears to be ready to sell'. Find frequency of the letter 't'

9