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

[Q]: Sentence: 'a passenger bus that is driving down the street'. Find frequency of the letter 'r'
[A]: 3


[Q]: Sentence: 'a man and a woman are walking down the street with an umbrella over their heads'. Find frequency of the letter 'w'
[A]: 4


[Q]: Sentence: 'a skateboarder in the middle of performing a trick'. Find frequency of the letter 'a'
[A]:
4