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

Sentence: 'a girl hitting a tennis ball on a court'. Find frequency of the letter 'l'
3

Sentence: 'a silver train crossing the road on tracks'. Find frequency of the letter 'r'
5

Sentence: 'a brick clock tower with an ornate clock'. Find frequency of the letter 'o'

4