Goldman Sachs Interview Question: You are in a room with 3 swit... | Glassdoor

Interview Question

Candidates Day Interviews for Multiple Areas (X-Div/Back Office) Interview New York, NY

You are in a room with 3 switches which correspond to

  3 bulbs in another room and you don't know which switch corresponds to which bulb. You can only enter the room with the bulbs once. You can NOT use any external equipment (power supplies, resistors, etc.). How do you find out which bulb corresponds to which switch?
Tags:
brain teaser, logic, case
Answer

Interview Answer

6 Answers

32

Turn on 2 switches.
Turn one switch off after a few minutes.

Enter room with bulbs.
The bulb that is on is the only switch that is on.
The bulb that is off but hot, is the switch that has been turned off.
The bulb that is off and cold is the third switch.

Interview Candidate on Mar 18, 2009
5

Turn on switch A for 1 minute. Turn it off.
Turn on switch B.
Enter the room with the bulbs.
The hot bulb is A
The lit bulb is B
The dark bulb is C

Someone on Oct 26, 2009
1

Turn switch A leave it on for 1/2 minutes and turn it off then turn on switch B. Now goto the room, the one Hot is A, Cold(or not Hot) is C, the bulb thats on is B.

recentone on Apr 8, 2010
2

The answers here all have two assumptions which are unwarranted.
1. Why do you think the light emits heat?
2. Why do you think you can touch the bulbs?
(3. If you're never going to enter the room again what's the point of knowing which switch goes with which light?)

The best you can do is reduce to guessing one of two bulbs.

Objection on Oct 26, 2010
1

RE: Objection
1. I guess you've never touched the outside of a light bulb after it's been on for awhile. The filament gets hot-it glows, the light we see is that glowing-the heat from from the glowing filament transfers to the glass-the glass is hot.

2. Why not? This is an exercise to demonstrate problem-solving ability, not a "MacGyver" episode. Who's to say the candidates didn't ask about the room's dimensions or where the bulbs were located, or that assumptions could be made in order to formulate an answer?

3. I think you missed the point of the question.

4. Sheesh.

3.

Cranky Old Guy on Dec 16, 2010
1

Yes, Objection, I did see that I entered "3." on here a second time. BTW, I'm not returning to this forum again, so any reply/retort/threat won't be seen by me. Good luck in your job hunting.

Cranky Old Guy on Dec 16, 2010

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