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

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


[Q]: Sentence: 'a man riding a skateboard up the side of a ramp'. Find frequency of the letter 'u'
[A]: 1


[Q]: Sentence: 'a man riding a train with a bicycle'. Find frequency of the letter 'n'
[A]:
3