[From the notes of Emily Moore. Modified slightly by Sam Rebelsky.]
A contractor estimated that one of her two bricklayers would take 9 hours to build a certain wall and the other 10 hours. However, she knew from experience that when they worked together, 10 fewer bricks were laid per hour. Since she was in a hurry, she put both workers on the job and found it took exactly 5 hours to build the wall. How many bricks did the wall contain?