Consider the question. On Friday, Hank opened his used-bicycle store with a stock of bicycles, all fixed-up and ready to ride. Over the course of the day, he sold 10 bicycles and bought an additional 15 to fix up. On Saturday, he sold 12 bicycles and bought 8 more. And on Sunday, he sold 9 bicycles and bought 11 more. What was the net increase in the number of bicycles in stock in Hank's store over the three days?
 If the answer is '3'; explain the reasoning:
He sold 10 + 12 + 9 = 31 bicycles. He bought 15 + 8 + 11 = 34 bicycles. He now has 34 - 31 = 3 more than at Friday's opening.