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

Sentence: 'a group of people having dinner and drinking wine'. Find frequency of the letter 'r'
3

Sentence: 'there is no image here to provide a caption for'. Find frequency of the letter 'o'
5

Sentence: 'a black and white photo of three baseball players '. Find frequency of the letter 'a'

6