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

Sentence: 'a bunch of cows that are in the field'. Find frequency of the letter 'a'
3

Sentence: 'a bird outside standing on a table looking up'. Find frequency of the letter 'b'
2

Sentence: 'a tennis player rushes to hit a tennis ball'. Find frequency of the letter 'n'

4