MATH PUZZLE: Can you replace...

One night, a man on his way home happened upon a drunk, down on his hands and knees searching for something under a street light. The man asked the drunk what he was looking for so diligently and the drunk said he had tripped and his Rolex wrist watch had broken loose from his wrist. The man, being a kindhearted soul, got down on his hands and knees and began assisting the drunk looking for his watch. After about ten minutes without any success, the man asked the drunk exactly where he tripped. "About a half a block up the street," the drunk said. "Why, pray tell," the man asked the drunk, "are you looking for your watch here if you lost it a half a block up the street?" The drunk replied, "The light is a lot better here."