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

done in 3 weighings.... I think =P by Vina)

But how can you be sure that the first weighing is 2+3 v 5 or 2+5 v 7. It would take at least one more weighing beforehand.