Virtu Financial Interview Question: one bulb in a sealed room, th... |

Interview Question

Trading Operations Analyst Interview London, England

one bulb in a sealed room, three switches, a,b,c, outside

  of the room, the room is totally sealed, nothing can be seen even the bulb is on, you can enter the room only once, how to confirm which switch is for that bulb

Interview Answer

2 Answers


put a to on for 30min, then put it back to off, put b to on, then immediately enter the room: if bulb is off, touch it, if it is warm, switch a is for the bulb, if it is cold switch c is for the bulb; if the bulb is on, switch b is for the bulb.

Interview Candidate on 27 May 2015

Here is a similar but maybe better strategy: put a and b on for sometime, then put a off. Go in and see if bulb is on. If it is on, switch b is the correct one. If it is off and warm, switch a is the correct one. If neither of the above two cases, switch c is the correct one.

Anonymous on 27 Jan 2016

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