Weighing problem
There are 27 coins, each of them weigh 10 g, except for one. The one different coin weighs either 9 g or 11 g (i.e., lighter or heavier by 1 g). You have a balance scale. What is the minimum number of weighings you need to always guarantee the determination of the different coin?
1 comment:
4 weighings.
Divide them in 3 groups of 9.
1) Compare 1st group and 2nd group.
Then
2) compare 1st group and 3rd group.
Now you know which group has the different coin. You also know whether the coin is heavier or lighter. Divide that group in 3 groups of 3.
3) Compare any 2 groups.
Now you know which group of three has the different coin.
4) Compare any two coins of that group.
You have the bad coin.
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