Measuring Kilograms Puzzle

The Puzzle:

Measuring Kilograms
In front of you is a balance.

Can you devise a system, using only 4 weights, that can measure any whole number of kilograms (1,2,3, etc) ...

a) ... up to 15 kg ?
b) ... up to 40 kg ?
c) ... up to 80 kg ?
Do you have the answer? Check against our solution!