Logical Puzzles Medium 3 min

Three switches are outside a closed room. Inside the room is one light bulb. Only one switch controls the bulb. You may manipulate the switches as much as you want, but you may enter the room only once. How can you identify the correct switch?

Hint

The bulb gives information through both light and heat.

Answer

Turn switch 1 on for several minutes, then turn it off. Turn switch 2 on and enter the room. If the bulb is on, switch 2 controls it. If it is off but warm, switch 1 controls it. If it is off and cool, switch 3 controls it.

Explanation

The bulb provides two observable states besides being lit: warm or cool.

Step-by-Step Solution

1. Turn switch 1 ON.
2. Wait several minutes.
3. Turn switch 1 OFF.
4. Turn switch 2 ON.
5. Enter the room.
6. If the bulb is ON, switch 2 controls it.
7. If the bulb is OFF but warm, switch 1 controls it.
8. If the bulb is OFF and cool, switch 3 controls it.

6 puzzles

Six people A, B, C, D, E and F sit in a row. A sits immediately to the left of B. C sits immediately to the right of D. E sits at the extreme left. F sits at the extreme right. Who occupies the two middle positions?

18 views Logical Puzzles

Medium

Five students P, Q, R, S and T scored different marks. P scored more than Q but less than R. S scored less than Q. T scored more than R. Who scored the highest and who scored the lowest?

15 views Logical Puzzles

Hard

Four tasks W, X, Y and Z must be completed one at a time. W must be before X. Y must be before Z. X must be before Y. Which task must be completed third?

26 views Logical Puzzles

Hard

Seven runners finish a race. A finishes before B. B finishes before C. D finishes after C. E finishes before A. F finishes after D. G finishes before E. Who must finish before everyone else?

17 views Logical Puzzles