In a market when the price of a good changes:
O both the demand curve and the supply curve for the good shift.
O demand for the good changes.
O both demand for and supply of the good change.
O both quantity demanded and quantity supplied change for the good.

O both quantity demanded and quantity supplied change for the good.

Explanation:

A change in the price of a good leads to a movement along the demand and supply curve for that good. This leads to changes in both quantity demanded and quantity supplied for the good.

All things being equal, the higher the price of a good, the lower the quantity demanded and the lower the price of a good, the higher the quantity demanded.

All things being equal, the higher the price, the higher the quantity supplied and the lower the price, the lower the quantity supplied.

Other factors other than changes in price of the good lead to changes in both demand for and supply of the good and shifts both the demand curve and the supply curve for the good.

I hope my answer helps you

u = 30

continuing your method

= +

subtract from both sides

= - thus

= -

= ⇒ u = 30