Given a math word problem, answer the following question. You might need to apply addition or subtraction mathematical operators on the numbers embedded in the text to answer the question and then only report the final numerical answer.

[Q]: harry hound had a terrible earache yesterday . when i peered into his ears yesterday i found 36 frisky fleas having a party in his right ear and 85 baby fleas sleeping peacefully in his left ear . i cleaned out harry hound 's ears . how many fleas perished ?
[A]: 121

[Q]: a company was offering a special on cell phones for $ 2 each . but only if you spent 7 dollars a month for 4 months . how much would it end up costing you total if you bought 1 phone ?
[A]: 30

[Q]: a waiter had 19 customers . after some left he still had 4 customers . what 's the difference between the number of customers before leaving and after leaving ?
[A]:
15