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

Sentence: 'a dog has found a rather large stick for playing fetch'. Find frequency of the letter 't'
3

Sentence: 'two people under an umbrella while walking down a sidewalk'. Find frequency of the letter 'i'
3

Sentence: 'a group of skiers posing for a photo'. Find frequency of the letter 'p'
3