How much of this good would be demanded and supply if it was given away free of cost? Find demand and supply when p is 0.

Substitute $P=0$ into the demand equation:

$$Q_D = 100 - 10(0)$$ $$Q_D = 100 - 0$$ $$Q_D = 100$$

Substitute $P=0$ into the supply equation:

$$Q_S = 40 + 20(0)$$ $$Q_S = 40 + 0$$ $$Q_S = 40$$

At a price of $0, the quantity demanded is 100 units and the quantity supplied is 40 units.

Substitute $P=4$ into the demand equation:

$$Q_D = 100 - 10(4)$$ $$Q_D = 100 - 40$$ $$Q_D = 60$$

Substitute $P=4$ into the supply equation:

$$Q_S = 40 + 20(4)$$ $$Q_S = 40 + 80$$ $$Q_S = 120$$

At a price of $4.00, the quantity demanded is **60 units** and the quantity supplied is **120 units**. Since$Q_S > Q_D$, there is a surplus.

$$\text{Surplus} = Q_S - Q_D = 120 - 60 = 60 \text{ units}$$

There is a surplus of 60 units.