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

Sentence: 'a train station with a train pulling into the platform'. Find frequency of the letter 'i'
6

Sentence: 'a woman walking on a shore next to a river'. Find frequency of the letter 'v'
1

Sentence: 'four young skateboarders are holding onto the back of a bus'. Find frequency of the letter 'o'

7