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

Sentence: 'an old woman is holding her cat in front of a camera'. Find frequency of the letter 'r'
3

Sentence: 'a passenger train that is going over a bridge'. Find frequency of the letter 't'
3

Sentence: 'two women standing beneath a white and blue umbrella'. Find frequency of the letter 'g'
1