I'm probably making a really stupid error (I often make those in numerical contexts!), but I'm having a bit of trouble with an introductory economics assignment that involves a little elementary algebra. BACKGROUND: We learned about the supply-demand equilibrium - a price at which the quantity supplied equals the quantity demanded. QUESTION: Suppose the demand & supply for apples is given by: Demand: QD = 100 - 2P Supply: QS = 50 + 3P a) Solve for the equilibrium price and quantity for apples. MY SOLUTION: Well, I made both equations equal & solved for the unknowns: 100 - 2P = 50 + 3P 50 = 5P P = 10 Since 50 + 3(10) and 100 - 2(10) both equal 80, then QD = QS = 80 And it seems all nice & what not, but then the next sub-question asks: "With price on the vertical axis & quantity of apples on the horizontal axis, graph the demand & supply curves & illustrate the equilibrium price and quantity". So, to try finding some points on the curve I tried to find the price at which demand would equal zero. QD = 0 = 100 - 2P 2P = 100 P = 50 BUT 50 IS A LOWER PRICE THEN 80! And I was taught that the demand of a good goes up as it's price goes down, so this makes no sense. At 50, their should be more demand that at 80, NOT LESS! Am I confusing something here?