You have N coins, 2 of which are radioactive. You have a radioactivity detector which can test any subset of the coins at a time, and return the number of radioactive coins in the group (i.e. it returns 0, 1 or 2). You have to find the radioactive coins, using not more than 10 tests. What is the largest N for which this is possible?

(In reply to A better solution by Joel)

Joel, I tried to improve on your solution but was not able to get above the 108 that you listed. I will be suprised if a higher number is found. L.L.

Posted by Leming on 2006-10-18 15:43:46