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?

I think I now have a solution for 120 coins (factor of 3 on one side, 5 on other in 3 tests) = 15x15 in 6, 75x45 in 9 = 120 total in 10.

I believe that the final solution may be 144 because it seems like the fibonnacci sequence (but that just be me seeing patterns that don't exist)

I'll post more if I either get better or give up

Posted by Joel on 2006-10-19 12:14:51