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
New high by Joel)
120 looks good, though it took me a while to get the 3 tests to cover all possibilities.
If the piles are divided into (a,b,c) and (d,e,f,g,h) then:
First test: ade Answer: 2 then test d
Answer: 0 then test bf followed by bg
Answer: 1 then move to Second test: aefg
Second test: aefg Answer:2 then test f
Answer: 0 then test b
Answer: 1 then test be
Edited on December 8, 2006, 10:18 am

Posted by Leming
on 20061019 15:50:48 