Traps and Tips:

There are differences between “Demand” and “Quantity of Demand”, “Supply” and “Quantity of Supply”. Please be very careful when reading the questions.

Usually, “Demand” and “Supply” means the Demand Curve and Supply Curve. So an increase in Demand means that the Demand Curve is shifted to the right, while an increase in Supply Curve means that it is shifted to right. The change in Demand and Supply (i.e. shifting of these curves) occurs when, for example, technology is changed, substitution goods appear and customer taste changes.

But the change of Quantity of Demand and Supply is just referring to the movement along the Demand and Supply Curve. For example, when the price increases, the Quantity of Demand will decrease and the Quantity of Supply will increase. This is always true (unless it is perfectly inelastic) regardless of the changes of the factors (technology, substitution etc) mentioned above.

Usually the questions will mention only Demand and Supply. It means the Curves NOT the quantity of the demand and supply!

E.g. If the demand of milk increases, and assume supply being the same, how would the price change?

The price will increase! This is because the Demand Curve is shifted to the right and intersects the supply curve at a higher price. This is not about the quantity of demand, which will make you draw the conclusion that the price will decrease!