“You guys will love this place,” Jesse’s grandfather said. “I go there all the time.”
Jesse and his friend Thomas were visiting Jesse’s grandfather, who loved boating and had just bought a new boat. From Jesse’s grandfather’s house along a river, they would be going downstream to a park that had a fishing pier and a restaurant.
“How far is it?” Thomas asked as they got on the boat.
“20 nautical miles,” Jesse’s grandfather said. “A nautical mile is about 1/6 longer than a land mile, or almost 2 kilometers.”
The boat’s speedometer showed the engine was running at 20 knots, which Jesse’s grandfather said meant 20 nautical miles per hour on the trip there, which lasted 48 minutes. They had a fun day, although they didn’t catch any fish. The speedometer showed the same speed on the trip back, which took 1 hour and 20 minutes.
Thomas and Jesse climbed out onto the dock while Jesse’s grandfather tied up the boat.
“Well, we learned something today,” Thomas said to Jesse.
“You mean that we’re no good at fishing?” Jesse said.
“Not that. If the distance was 20 nautical miles and the boat was moving at 20 knots, the ride should have taken 1 hour, but it didn’t—in either direction. Either the speedometer is wrong about the speed, or your grandfather is wrong about the distance,” Thomas said. “But which?”