Wordplay, the New York Times Crossword Blog, doesn’t just post info about crossword puzzles. They also run a regular feature on mathematics and logic problems called Numberplay, which is quite fun.
Here’s a puzzle that appeared recently on Numberplay … I didn’t get it, but my 14-year-old son did in just a few minutes:
You are in a room with three switches each of which controls one of three table lamps in a separate room thatâ€™s some distance away. The switches are all in the â€œoffâ€ position and have been so overnight, but you know that all three table lamps work. Once you leave the switch room, you can go into the room with the lamps only once. How do you figure out which light switch goes with which table lamp? No tools, helpers or cellphones allowed.
Here’s the solution:
[spoiler /Show Answer/ /Hide Answer/]Turn off switch 1, wait 5 minutes, turn off switch 2, then go into the lamp room. The lamp that is on is controlled by switch 3. Now feel the two bulbs of the off lamps – the hotter of the two off lamps is controlled by switch 2, and the cooler of the two off lamps is controlled by switch 1.[/spoiler]