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

Sentence: 'a person on a skateboard going down a ramp'. Find frequency of the letter 'n'
4

Sentence: 'a construction sign is displayed on a street'. Find frequency of the letter 'i'
4

Sentence: 'a multi-colored transit bus parked next to a sidewalk'. Find frequency of the letter 'l'

3