There are two types of consumers of potatoes: 10 are rich and 50 are poor. Every rich consumer has a demand of 0 for a price of $500/ton; and his/her demand decreases by 1 ton each time the price increases by $1/ton. Every poor consumer has a demand of 0 for a price of $280/ton; and his/her demand decreases by 2 tons each time the price increases by $1/ton. Interpret the differences between the demand of the rich and the demand of the poor. What is the total market demand for potatoes? Give the equation of the demand curve Q = D(P)?