Measuring Marbles

by | Brain Teasers

This month we have another classic-style brain teaser. This one requires some creative problem solving. The answer is a surprisingly small number, so give yourself a chance to work it out and don’t jump right to the answer!

You have 9 marbles. 8 have the same weight and 1 is slightly lighter. You also have a balance scale. What is the minimum number of times that you need to use the balance scale to determine which is the lighter marble?


Answer:


The answer is 2. Divide the 9 marbles into 3 groups of 3. Compare the weight of 2 these 3 groups. If one group is lighter, then the marble is in that group. If the 2 groups are equal, then the lighter marble is in the group that wasn’t weighed. Now choose 2 marbles from the group that has the lighter marble and weigh them. If one is lighter, that is your marble, if not, then it is the one that wasn’t weighed.