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Interviewer has given me 100 marbles(50 white and 50 black) and two empty boxes.............So whats should i have done?

+2 votes

This was asked in an interview of Infosys.
Interviewer has given me 100 marbles(50 white and 50 black) and two empty boxes.
He then told me that he will leave the room and i need to place all the marbles in two boxes.

And When he come back, he will draw a marble from any of the two box and if the marble is black i will be hired.

* No box can be empty.
* All 100 marbles must be placed in one of the two boxes.

So whats should i have done ?

posted Apr 14, 2018 by Pankaj Deshmukh

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1 Answer

+1 vote

If there are no cheap tricks to this then it becomes a problem of increasing the probability of the interviewer picking a black marble to the maximum.

This can be done by placing 1 black marble on one box and all the rest of the 99 marbles on the 2 box.

The probability if the 2 boxes had equal number of black and white marbles: 0.5×0.5 + 0.5×0.5 = 0.5 = 50%
The probability when 1 box has 1 black marble in one of the boxes and 99 in another: 0.5×1 + 0.5×(49/99) = 0.747474 = 74.7474%.

answer Apr 14, 2018 by Tejas Naik

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