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

Sentence: 'young boy flying kite in large field at outdoor event'. Find frequency of the letter 'y'
3

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

Sentence: 'two young women from opposing teams tangle while going for a soccer ball'. Find frequency of the letter 'n'

5