Give the step-by-step reasoning process and then the final answer. As Dan is learning to screen-print t-shirts to sell at the craft fair, he makes t-shirts, over the first hour, at the rate of one every 12 minutes. Then, in the second hour, he makes one at the rate of every 6 minutes. How many t-shirts does he make over the course of those two hours?
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He made 60 / 12 = 5 the first hour. He made 60 / 6 = 10 the second hour. He made 5 + 10 = 15.
The final answer: 15.
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