Mind Bender # 4 : Solution

This Mind Bender always draws some good responses – ranging from …

  • Place one bag on the scales at a time, until you discover the lighter one; and
  • Put all bags onto the scales, and remove them one at a time for the result; and
  • Assuming it is a balancing scale: Put five bags on each side, then remove two at a time to find the lighter bag; through to …
  • Simply lift each bag manually to “feel” which bag is lighter, and avoid the need to use the scales at all.

But as you’ll recall … the only RULE was is that there was to be just ONE weighing. Therefore, by adding (or subtracting) bags from the scales … you would actually be making multiple weighings – which is not permitted.

And, unless you work with weights and measures for a living … I’m not sure how accurate a “manual weighing” would prove.

So, here is how you do it …

First, you number the bags from 1 to 10.

Then take from each bag a number of coins equal to the number on the bag [ie: one coin from bag No. 1, two coins from bag No. 2, and so forth].

Now weigh them all together – where a total should be 550 grams.

However, the result will be lighter by a number of grams equal to the number written on the bag containing the lighter coins.