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

Sentence: 'a view from a bathroom shows an open door'. Find frequency of the letter 'o'
7

Sentence: 'a man and several white red and blue airplanes'. Find frequency of the letter 'u'
1

Sentence: 'a teenager enjoying snowboarding down a hill on a cold day'. Find frequency of the letter 'l'
3