MATH PUZZLE: Can you replace...

The strong young man at the construction site was bragging that he could outdo anyone in a feat of strength. He made a special case of making fun of one of the older workmen. After several minutes, the older worker had had enough. "Why don't you put your money where your mouth is," he said. "I will bet a week's wages that I can haul something in a wheelbarrow over to that outbuilding that you won't be able to wheel back." "You're on, old man," the braggart replied. "Let's see what you got." The old man reached out and grabbed the wheelbarrow by the handles. Then, nodding to the young man, he said, "All right. Get in."