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

Sentence: 'a man is water skiing on a river'. Find frequency of the letter 'r'
3

Sentence: 'a woman swinging a tennis racquet towards a tennis ball'. Find frequency of the letter 'e'
3

Sentence: 'two road signs, one saying dead end and one saying it is a road'. Find frequency of the letter 's'

5