MATH PUZZLE: Can you replace...

Bill, Jim, and Scott were at a convention together and were sharing a large suite on the top of a 75-story skyscraper. After a long day of meetings they were shocked to hear that the elevators in their hotel were broken and they would have to climb 75 flights of stairs to get to their room. Bill said to Jim and Scott, let's break the monotony of this unpleasant task by concentrating on something interesting. I'll tell jokes for 25 flights, and Jim can sing songs for 25 flights, and Scott can tell sad stories the rest of the way. At the 26th floor Bill stopped telling jokes and Jim began to sing. At the 51st floor Jim stopped singing and Scott began to tell sad stories. "I will tell my saddest story first," he said. "I left the room key in the car!"