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

Sentence: 'a group of tennis players on a tennis court'. Find frequency of the letter 'o'
4

Sentence: 'a small wrinkley dog wearing a bow tie'. Find frequency of the letter 'e'
3

Sentence: 'a black and white photo of a skateboarder doing a trick'. Find frequency of the letter 'r'

3