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

Sentence: 'a red and white train passing a train station'. Find frequency of the letter 'a'
7

Sentence: 'a person working at an airport, outside of an airplane'. Find frequency of the letter 'r'
5

Sentence: 'outside of a tennis court are a man kneeling down while another man is in a seat sitting up high and a tennis player in blue is having their foot wrapped'. Find frequency of the letter 'n'
15