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 red fire hydrant opened and spilling water'. Find frequency of the letter 'd'
4

Sentence: 'there is a bathroom with a sink and toilet'. Find frequency of the letter 'i'

4