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Radioactive Coins (Posted on 2006-10-17) Difficulty: 5 of 5
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?

No Solution Yet Submitted by David Shin    
Rating: 4.5000 (6 votes)

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Some Thoughts A little bit closer | Comment 10 of 18 |
(In reply to re(2): New high - Marvelous by Joel)

some code tweaks and a better machine got me to:

8 : 65

(i.e. 66 coins requires 9 tests and 65 can be done in 8)

which is changing my projections slightly to mid 190s, possibly low 190s

Since that is about 3 times where I've gotten so far and I think my code is using exponentially more time as it progresses, I don't have much hope for actually solving the problem directly via this code.  Still, it is gratifying to get a little closer.  I've got a few more algorithmic improvements to make.

  Posted by Joel on 2006-10-20 17:48:05

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