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

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

Sentence: 'an orange cat sitting on the ground next to a  purse'. Find frequency of the letter 's'
2

Sentence: 'a doorway looking into a small bathroom with white toilet'. Find frequency of the letter 'r'

2