Switches
Written on 12:43 am by Vja Students
Four switches can be turned on or off. One is the light switch for the incandescent overhead light in the next room, which is initially off, but you don't know which. The other three switches do nothing. From the room with the switches in it, you can't see whether the light in the next room is turned on or off. You may flip the switches as often and as many times as you like, but once you enter the next room to check on the light, you must be able to say which switch controls the light without flipping the switches any further. (And you can't open the door without entering, either!) How can you determine which switch controls the light?
Ans: Turn on switches 3 and 4 and wait fifteen minutes or so. Then turn switch 3 off, turn switch 2 on, and enter the room. If the bulb is dark and cool, switch 1 controls it. If the bulb is bright and cool, switch 2 controls it. If the bulb is dark and warm, switch 3 controls it. If the bulb is bright and warm, switch 4 controls it.
Let the switches be A,B,C,D.
1. Switch on A and B and wait for a while
2. Switch off A and switch on C
Now immediately go into next room and check the bulb by watching and touching it.
If the bulb is
Off and hot - Switch A
On and Hot - Switch B
On and Cold - Switch C
Off and Cold - Switch D.
good brain teaser
Suppose there is Switch A, B, C, D and all are Off.
First on A for 5 min .
Now off A and after 5 min on B for 5 min.
Now off B and on C.
Get in to the room.
if light in on - Ans in C.
If light bulb is off then touch the bulb and if its extrem hot then ans is B.
if its not much hot then ans is A.
and if its not at all hot then ans is D.