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

Sentence: 'a large cooked pizza on a dining table'. Find frequency of the letter 'n'
3

Sentence: 'a group of cows that are standing in the grass'. Find frequency of the letter 'g'
3

Sentence: 'a tabby cat standing next to an open umbrella on the floor'. Find frequency of the letter 'a'

6