Guess The Age

A dottering, old professor of logic asked his College class a question.
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"If Philadelphia is 100 miles from New York and Chicago is 1000 miles from Philadelphia and Los Angeles is 2000 miles from Chicago, and the Moon is 239,000 miles from Earth, how old am I?"
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A student in the back of the class raised his hand and when called upon said, "Professor, you're 70."
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The old professor said, "You're absolutely correct, but tell me, how did you arrive at the answer so quickly?"
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The student said, "It's easy, I have a brother, he's 35, and he's half nuts."