Wednesday, February 02, 2011

Lessons

Oh well, I've learnt my lesson- always beware of Geeks bearing gifts.

Here's a puzzle somebody told me today- first one to solve it wins a holiday to Egypt.

There are three matchboxes in front of you. One contains two black marbles, the second, two white marbles and the third one has a black and a white marble inside. Each box had the correct label on it until some pesky kid switched them around so that each matchbox is now wrongly labelled.

You can take one marble at a time from any box without looking inside. How many marbles must you remove to be sure of correctly identifying all 3 boxes?

6 comments:

Anonymous said...

One from the box marked both

IceQueen

paulsc said...

I agree.
If the boxes are arranged as follows then there are only two arrangements for the labels

2 white.....2 black.....1 of each

.2B..........1B1W.........2W

.1B1W........2W...........2B

As the IceQueen said take one from the box marked both, or in my case 1B1W, and this will identify the label arrangement.
Assume I do this and I retrieve a white ball then I know that it is the second arrangement. This means that I can identify the pair of black marbles by the label that says 2W and the one of each box by the 2B label.
Ignore the dots they are just there to keep the characters apart since multiple spaces are compressed into one.

CSI Guy said...

Holiday to Egypt..haha!

Anonymous said...

Two tries. As the boxes contain either two black, two white, or one black and white marble.

"has a black and a white marble inside."

So it only takes one try to identify any box. After two tries you know all three boxes.

Anonymous said...

Oops. Fail. Didn't read Q properly :-(

Jonathon said...

Remove???

There won't be any left to remove after the kids have had them.....