Thread: Smallest number of items from given probability

Hey All,

Another problem I'm confused with.

A bag contains several blue discs and several yellow discs. Find the smallest number of discs which should be taken out of the bag, if the probability that at least 3 of those removed are of the same color is 1.

Only thing I can see is that least number must be > 3. How do I proceed if the no of items is unknown. I have feeling I am missing something simple, a hint would be great.

Thanks again for all your help!

its hard to give a useful hint that isn't the answer itself. Try thinking along these lines:

Suppose i took out 2 discs, it it possible that no 3 of them are the same color?
Yes, (obviously since there aren't 3 discs!). eg 2 could be blue, none yellow.

Suppose i took out 3 discs, it it possible that no 3 of them are the same color?
Yes, eg 2 could be blue, one yellow (or the other way around)

Keep going until you find that it is impossible (it wont take long!)

5! either way will have 3 of same color. Thanks, seems so simple now that you showed the way.