Question 52464: sam had 9 marbles that all looked alike, but 1 was a little heavier. He then put the same number of marbles on each side of a balance scale and found the heavier marble by using the scale twice. How did he do it? (the second try need not have the same no. as the first)
Answer by Earlsdon(6294)   (Show Source):
You can put this solution on YOUR website! Sam probably divided the 9 marbles into three groups of three each.
Then he put aside one of the groups and weighed the other two groups, three marbles on each side.
1st weighing: If the scale balances, Sam now knows that the heavier marble is in the group of three that he put aside. So now he takes that group, puts one marble aside and weighs the remaining two marbles.
2nd weighing: If the scale balances, then Sam knows that the heavier marble is the one he put aside and he is done.
If the scale doesn't balance, then Sam has found the heavier marble on the scale.
If the scale doesn't balance on the 1st weighing, then Sam takes the three marbles from the heavier side, puts one of them aside and weighs the other two.
2nd weighing: See above for the result of the 2nd weighing.
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