Use the 50g weight to weigh off a 50g lump of clay, and do the same with the 20g weight. Put the 20g on one side of the scale, and the 50g on the other. Remove some clay from the 50g lump until the scale balances. Now take the clay that u removed from the 50g lump. You should have 30g of clay. Break it into two lumps, and use only those two lumps to make the scale balance. When it balances, you will have two 15g lumps.