1= 1

2=3- 1 (the minus sign indicates the weight of 3g on the right and the weight and object of 1g on the left, so the weighed object is 2g, and the minus sign below is similar).

3=3

4= 1+3

5=9- 1-3

6=9-3

7=9+ 1-3 (put 1, 9 on the right, 3 and the object on the left, you can call it 7).

8=9- 1

9=9

10=9+ 1

1 1=9+3- 1

12=9+3

13=9+3+ 1