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

Sentence: 'a red fire hydrant opened and spilling water'. Find frequency of the letter 'd'
4

Sentence: 'a row of vintage cars in a grassy field'. Find frequency of the letter 'v'
1

Sentence: 'a guy in a brown jacket standing near a sign holding a cellphone to his ear'. Find frequency of the letter 'n'

8