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

Sentence: 'a cat that is standing next to an open suitcase'. Find frequency of the letter 'e'
3

Sentence: 'a little dog on a table while a dog groomer cuts its fur'. Find frequency of the letter 'r'
3

Sentence: 'two adjacent passenger trains stopped at a station'. Find frequency of the letter 'd'

2