Four weights puzzle

Q: You need to weigh up to 40 kg with 4 weights. What should be the weigh of each of weight?
A: 1 kg, 3 kg, 9 kg, 27 kg
Solution :
1 : 1 kg
2 : (3 -1) Kg
3 : 3 Kg
4 : (3+1) Kg
5 : (9-(3+1)) Kg
6 : (9-3) Kg
7 : ((9+1)-3) Kg
8 : (9-1) Kg
9 : 9 Kg
and so on..