56. 10 $50 \mathrm{~g}$ weights and 5 $100 \mathrm{~g}$ weights are placed on both sides of the balance at the same time to keep the balance, then if 2 $1 \mathrm{~kg}$ weights are placed on the left side of the balance, and 6 $300 \mathrm{~g}$ weights are placed on the right side, how many $50 \mathrm{~g}$ weights need to be added to the right side to keep the balance?
Reference answer: 4