Impossipuzzle

“Fifty minutes to Jack’s place and back,” said Sam. “That’s good going on an old two-speed bike, even though you started back right away.”

Dick nodded. “It’s mostly downhill, Dad, so I averaged 12 miles an hour going there,” he declared. “But coming back I could only make 8 miles an hour. I timed it both ways just for fun.”

How far was Jack’s place?