You have 4 weights weighing 2,3,5 and 7 pounds. The problem is none of them are marked. What is the fewest number of weighings you need using a balance scale figure out which weights are which?
(In reply to re(2): done in 3 weighings.... I think =P
Your method is dependant on the initial weighing. If the initial weighing is not balanced, then your method fails. A proper solution must account for all possible results of the first weighing.