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

Sentence: 'a man sitting on a motorcycle near two other men'. Find frequency of the letter 'r'
3

Sentence: 'an image of a street sign with a corner restaurant'. Find frequency of the letter 'u'
1

Sentence: 'a colorful umbrella stands apart from a black and white photo of people standing in front of a statue'. Find frequency of the letter 'n'
6