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

Sentence: 'a man flying through the air on a bicycle'. Find frequency of the letter 'u'
1

Sentence: 'a young man riding a skateboard near a forest'. Find frequency of the letter 'd'
2

Sentence: 'the corner post of a bed that is made up'. Find frequency of the letter 'e'
4