What was the question for this implicit rationale, and corresponding answer?
He gets 3 * $400 = $1200 per month. So he gets 1200 * 12 = $14,400 per year. So he pays 12 * 900 = $10800 a year. That means his profit is 14400 - 10800 = $3600 per year.
 The answer: 3600
The question: John sublets his apartment to 3 people who each pay $400 per month. He rents the apartment for $900 a month. How much profit does he make in a year?