The following question was presented:
Brain teaser time to get you going. You have nine cookies. Eight are identical. The ninth is identical with the exception that it is heavier as a gold ring was baked inside it. You have a set of balance scales (the ones that allow you to put something on one side and something on the other and tell you which is heavier). What is the fewest number of times you need to use the scale to determine which cookie contains the ring?
Results:
0 was the clear winner with 60% of the votes selected from the following:
- 0 (60%)
- 1 (20%)
- 2 (0%)
- 3 (20%)
- 4 (0%)
- 5 (0%)
- 6 (0%)
- 7 (0%)
- 8 (0%)
- 9 (0%)
Graphic of Results:
Analysis:
Interesting - nobody got this one right. The correct answer is two.
Divide the cookies into three groups of three. Put two of them on either side of the scales and pick the group that is the heaviest. If they are the same, then pick the group that is not on the scales. At this point we know that the ring is in one of the three cookies in the heaviest group. Simply do the same thing again - pick two of the cookies and put one on either side of the scales. If there is a heavier one then it contains the ring. If not, the ring is in the one not on the scales.