If you haven't done any of the other
coin problems, then you might want to go back and try those now, this one is very difficult, even if you have figured out the other ones.
This time, as the title implies, there are 39 coins, and one is fake. You have a balance scale, which can be used 4 times.
How would you find the fake coin?
(In reply to
THE SOLUTION by Jonny Doe)
The problem did not state that the fake coin weighs less than the others; it might weigh more instead.
Also, 2 weighings (the last paragraph remark) are enough to differentiate only 9 possibilities (3 possibilities for result of first weighing x 3 possibilities for second).

Posted by Charlie
on 20030423 18:21:50 