Tuesday Tangle – Much Ado about Lighting

Today’s Tangle doesn’t involve any mathematics, but does require lateral thinking (I was hopeless at it, but Handsome Husband triumphed). I’ve included the answer in today’s post, for those who just can’t wait until tomorrow.

While renovating a house, Meryl discovers three light switches, side by side, on the second floor. When she asks the builder what the switches are for, she learns they control the three bare bulbs that hang in the attic, one floor above. He can’t, however, tell her which bulb is controlled by which switch.

Meryl, being lazy, wants to climb the attic stairs as few times as possible to solve the mystery. Is there a way she can determine which light switch controls each light bulb, with only ONE TRIP up to the attic?

(Solution below)

Solution (roll over with mouse to reveal): Yes, there is a way she can do it with only one trip to the attic. 

Let’s call the switches A, B and C. Meryl switches on light switch A, and leaves it on for five minutes. After five minutes, she switches A off, and switches on B.

Then she goes up to the attic. The bulb which is lit is obviously controlled by switch B. If Meryl touches the remaining two (unlit) bulbs, one will be warm and one will be cold. The warm one is then controlled by switch A, and the cold one by switch C.