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

[EX Q]: Sentence: 'a pizza is in a box on a table outside'. Find frequency of the letter 'u'
[EX A]: 1

[EX Q]: Sentence: 'two people under an umbrella while walking down a sidewalk'. Find frequency of the letter 'i'
[EX A]: 3

[EX Q]: Sentence: 'a boy on a yellow surf board on the water'. Find frequency of the letter 'r'
[EX A]:
3