TASK DEFINITION: In this task, you need to count the number of times the given letter appears in the given sentence.
PROBLEM: Sentence: 'two persons standing outside a moving sub train'. Find frequency of the letter 'a'

SOLUTION: 3

PROBLEM: Sentence: 'cars parked outside if a retail store'. Find frequency of the letter 'd'

SOLUTION: 2

PROBLEM: Sentence: 'a wooden swing attached to a tree near a river'. Find frequency of the letter 'o'

SOLUTION:
3