Question:
A bungalow has one of its rooms located on the first floor and there are three identical 100 watt electrical bulbs fixed in the room. Each bulb is connected to a specific switch located at the basement. There are only three switches in the basement. All the bulbs are switched off at present and you are also at the basement area. The first floor cannot be seen from the basement area. If you are allowed to use your common prudence, what is the minimum number of times that you will have to go from basement to first floor to identify which switch goes to which bulb?
A bungalow has one of its rooms located on the first floor and there are three identical 100 watt electrical bulbs fixed in the room. Each bulb is connected to a specific switch located at the basement. There are only three switches in the basement. All the bulbs are switched off at present and you are also at the basement area. The first floor cannot be seen from the basement area. If you are allowed to use your common prudence, what is the minimum number of times that you will have to go from basement to first floor to identify which switch goes to which bulb?
Show Hint
Combine light and residual heat: switch one bulb off after it has been on a while, then turn on a second switch, and check all three states in a single visit.
Updated On: Jul 16, 2026
- 3 times
- 20 times
- 1 time
- 6 times
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Verified By Collegedunia
The Correct Option is C
Solution and Explanation
Step 1: Use heat as a second clue, not just light.
Turn on switch 1 and let its bulb burn for about 5 minutes, then turn switch 1 off. Right away, turn on switch 2 and leave switch 3 off the whole time. Now walk up to the first floor. That is your only trip.
Step 2: Read the three bulbs at once.
The bulb that is glowing must belong to switch 2, since that is the only switch still on. Touch the two bulbs that are dark: the one that feels warm was recently lit, so it belongs to switch 1. The bulb that stays completely cool belongs to switch 3.
Step 3: Check why fewer trips cannot work.
With only one trip you can normally read just one property (on or off), which tells you at most one switch for sure. By adding the warm or cold clue, you pack a second property into the same visit, so one visit is enough to fix all three matches.
Final Answer:
You need to go up only once. \[ \boxed{1 \text{ time}} \]
Turn on switch 1 and let its bulb burn for about 5 minutes, then turn switch 1 off. Right away, turn on switch 2 and leave switch 3 off the whole time. Now walk up to the first floor. That is your only trip.
Step 2: Read the three bulbs at once.
The bulb that is glowing must belong to switch 2, since that is the only switch still on. Touch the two bulbs that are dark: the one that feels warm was recently lit, so it belongs to switch 1. The bulb that stays completely cool belongs to switch 3.
Step 3: Check why fewer trips cannot work.
With only one trip you can normally read just one property (on or off), which tells you at most one switch for sure. By adding the warm or cold clue, you pack a second property into the same visit, so one visit is enough to fix all three matches.
Final Answer:
You need to go up only once. \[ \boxed{1 \text{ time}} \]
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