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

Sentence: 'this yellow fire hydrant sits in a grass field'. Find frequency of the letter 'i'
5

Sentence: 'a yellow and green train on a train track in a wooded area'. Find frequency of the letter 'o'
4

Sentence: 'a truck with odd patterns and designs on it'. Find frequency of the letter 's'
3