You turn 2 switches "on" and leave 1 switch "off" and wait about a minute. Then enter the room, but just before you enter, turn one switch from "on" to "off". Once in the room, feel the lightbulb - if it is warm, but off, it has to be the last switch you turned off. If it is on, it has to be the switch left on. If it is cold and is off, it has to be the switch you left in the off position.
Before I left, I wound the wall clock. When I returned, the change in time equaled how long it took to go to my friends house and return, plus the time I spent there. But I knew the latter because I looked at my friends watch when I arrived and left.
Subtracting the time of the visit from the time I was absent from my house, and dividing by 2, I obtained the time it took me to return home. I added this time to what my friend watch showed when I left, and set the sum on my wall clock.
To see more past Problem of the Week's refer to the Riddles Newsletter annals.