You have 20 pounds of flour that must be distributed into 10 sacks of 2 pounds each.
At your disposal is a scale, and two weights: a 5 pound one and a 9 pound one.
How can you complete this job in only 9 weighings?
Weigh #1: Put the 9lb weight on one side of the scale, the 5lb weight on the other side, along with 4lbs of flour, so that the scale is even.
Weigh #2: take the weights off the scale, and transfer flour from one side to the other, giving you 2 piles of 2lbs flour on each side.
For weighings 3-8, repeat steps 1 & 2. This gives you a total of 8 sacks of flour, leaving you with 4 lbs of flour unweighed.
Weigh #9: put the rest of the flour on the scale. Adjust flour from one side to another until the scale is even, giving you the final 2 piles of flour, each of which must then be 2lbs.
different solution
