The 12 identicle ball (one odd ball) puzzle

Reply Tue 11 Apr, 2017 08:48 am
Find the odd ball among 12 almost identical balls The odd ball differs so minutely from the others that you have to use an accurate scale to find which one it is.

The solution to this puzzle can be found on the web but that would be cheating and taking all the fun out of mastering it yourself.

Using a two pan scale like the one usually found at a school of law

Or just use pen and paper to solve it without a scale

Try and solve this puzzle by finding the odd ball, with only 3 weighings

Here is the puzzle.

Problem : 12 balls are given. All but one are of equal weight/mass. You do not know whether the odd ball is lighter or heavier than the normal balls. You are given a comparison balance. You can use the balance scale only 3 times. How would you find out which is the odd ball? And whether it it heavier or lighter?

Have fun!!
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Reply Tue 11 Apr, 2017 12:46 pm
Spelling tip: identical.
Alan McDougall
Reply Tue 11 Apr, 2017 04:01 pm
centrox wrote:

Spelling tip: identical.

I know I know and I know, but the forum program will not let me go back and correct the spelling/keystroke error, which should read

The 12 identical ball (one odd ball) puzzle I am very embarrassed.
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Reply Wed 29 Nov, 2017 02:18 pm
Okay, here goes . . .

You weigh four balls against four balls. If they weigh up evenly, then you know that the odd ball is one of the remaining four. Of those remaining four balls, weigh two of them against two from the previous weighing. Let's say that the scale shows that the two balls from the previous weighing weigh less. That means that one of the other two balls weighs more.

Of those two balls, weigh one of them against a ball from the original weighing. If they weigh up evenly, then the other ball is the odd ball. If they don't weigh up evenly then the heavier ball is the odd ball.

However, if in the beginning you weigh four balls against four balls, and one side is heavier than the other, it seems that it would require four weighings to figure out which is the odd ball.
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