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

[EX Q]: Sentence: 'a silver train crossing the road on tracks'. Find frequency of the letter 'r'
[EX A]: 5

[EX Q]: Sentence: 'two people and two children walking in a building'. Find frequency of the letter 'd'
[EX A]: 3

[EX Q]: Sentence: 'a young girl holding a yellow tennis racquet'. Find frequency of the letter 'n'
[EX A]:
4