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

Sentence: 'a black computer with a screen reading "press any key to begin."'. Find frequency of the letter 'a'
5

Sentence: 'a person that is looking out his window'. Find frequency of the letter 'n'
3

Sentence: 'a meal is prepared of rice, broccoli, vegetables, and nuts'. Find frequency of the letter 'a'

5