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

Sentence: 'pedestrians pass by a stop sign, some holding surfboards'. Find frequency of the letter 'n'
3

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

Sentence: 'decorated baseball cake in a clear plastic box'. Find frequency of the letter 'k'
1