Four switches and a light bulb, the solution

If you don’t remember the puzzle from Paul Wilmott’s terrific book Frequently Asked Questions in Quantitative Finance, go back to the post "Solving fiendish problems"and yesterday’s hint. And if you tried and failed to figure it out, here’s the two-step solution. First, “turn on [switches] 1 and 2, and go and have some coffee.” Second, “turn off 1 and turn on 3, then go quickly into the room and touch the lamp.” (p. 384) If it is hot and dark it is controlled by switch 1, if it is hot and light it is controlled by switch 2, if it is cold and light it is controlled by switch 3, and—as by now even the worst puzzle solver should have figured out—if it is cold and dark it is controlled by switch 4.

A word of advice: if your brain seems to be controlled by switch 4 in dealing with such puzzles, just move on. Success is rarely found in a cold, dark world.