TASK DEFINITION: In this task, you need to count the number of times the given letter appears in the given sentence.
PROBLEM: Sentence: 'a big bear walks through some grass'. Find frequency of the letter 'a'

SOLUTION: 4

PROBLEM: Sentence: 'pedestrians pass by a stop sign, some holding surfboards'. Find frequency of the letter 'n'

SOLUTION: 3

PROBLEM: Sentence: 'two black cooling rack shoaling pieces of pizza'. Find frequency of the letter 'a'

SOLUTION:
4