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Take 3-3 balls and check weight, If equal use two left balls and check for light weight ball.

If 3-3 balls weight not equal take those 3 balls having less weight and take any two balls to check weight if equal weight then 3rd ball is light weight

Divide them into three groups, 2, 2 and 3.

Weigh 2 and 2, if equals, the lighter ball is in the last group. Take 2 out from 3 and weigh them, if equals, the left ball is the lighter ball, if not equal, take the lighter ball.
if not equal, the lighter ball is in the lighter group. Take the lighter group which contains 2 balls, weight them and take the lighter ball.


Another solution:

Divide into 3 groups, 3,3 and 1
Weigh the 3 and 3. If equal, the third group is the lighter ball.
If not equal, the ball is in the lighter group, which contains 3 balls. Take 2 balls out from the 3, weight 2 of them, if equal, the last ball is the lighter. If not equal, take the lighter ball.

1. Weigh a group of gp1 and gp2 of 3 balls each
2. if gp1 < gp2 weigh gp1.1 and gp1.2 if gp1.1 == gp1.2 return gp1.3 else return lighter ball
3. if gp1 == gp2, weigh gp3.1 and gp3.2

@ Anonymous on March 11, 2010

its 8 not 7..