The Problem Pool Balls  Solution
The Problem:
You have 12 balls identical in size and appearance but 1 is an
odd weight (could be either light or heavy).
You have a set scales (balance) which will give 3 possible readings:
Left = Right, Left > Right or Left < Right (ie Left and Right
have equal weight, Left is Heavier, or Left is Lighter).
You have only 3 chances to weigh the balls in any combination using
the scales. Determine which ball is the odd one and if it's heavier
or lighter than the rest. How do you do it?
The Solution:
Number the balls 1, 2, 3, ... 10, 11, 12
Start off with them in 3 groups: [1, 2, 3 and 4], [5, 6, 7 and
8] and [9,10,11 and 12]
Weigh 1, 2, 3 and 4 vs 5, 6, 7 and
8 with 3 possible outcomes:
1. If they balance then 9,10,11,12 have
the odd ball, so weigh 6,7,8 vs 9,10,11 with 3 possible outcomes: 

1a 
If 6,7,8 vs 9,10,11 balances, 12 is the odd ball.
Weigh it against any other ball to determine if heavy or light.


1b 
If 9,10,11 is heavy then they contain a heavy
ball. Weigh 9 vs 10, if balanced then 11 is the odd heavy ball,
else the heavier of 9 or 10 is the odd heavy ball. 

1b 
If 9,10,11 is light then they contain a light
ball. Weigh 9 vs 10, if balanced then 11 is the odd light ball,
else the lighter of 9 or 10 is the odd light ball. 



2. If 5,6,7,8 > 1,2,3,4 then either 5,6,7,8
contains a heavy ball or 1,2,3,4 contains a light ball so weigh
1,2,5 vs 3,6,12 with 3 possible outcomes: 

2a 
If 1,2,5 vs 3,6,12 balances, then either 4 is
the odd light ball or 7 or 8 is the odd heavy ball. Weigh 7
vs 8, if they balance then 4 is the odd light ball, or the heaviest
of 7 vs 8 is the odd heavy ball. 

2b 
If 3,6,12 is heavy then either 6 is the odd heavy
ball or 1 or 2 is the odd light ball. Weigh 1 vs 2, if balanced
then 6 is the odd heavy ball, or the lighest of 1 vs 2 is the
odd light ball. 

2c 
If 3,6,12 is light then either 3 is light or 5
is heavy. Weigh 3 against any other ball, if balanced then 5
is the odd heavy ball else 3 is the odd light ball. 



3. If 1,2,3,4 > 5,6,7,8 then either 1,2,3,4
contains a heavy ball or 5,6,7,8 contains a light ball so weigh
5,6,1 vs 7,2,12 with 3 possible outcomes: 

3a 
If 5,6,1 vs 7,2,12 balances, then either 8 is
the odd light ball or 3 or 4 is the odd heavy ball. Weigh 3
vs 4, if they balance then 8 is the odd light ball, or the heaviest
of 3 vs 4 is the odd heavy ball. 

3b 
If 7,2,12 is heavy then either 2 is the odd heavy
ball or 5 or 6 is the odd light ball. Weigh 5 vs 6, if balanced
then 2 is the odd heavy ball, or the lighest of 5 vs 6 is the
odd light ball. 

3c 
If 7,2,12 is light then either 7 is light or 1
is heavy. Weigh 7 against any other ball, if balanced then 1
is the odd heavy ball else 7 is the odd light ball. 



Puzzle supplied by Richard Rider
