At a unit price of $55, the quantity demanded of a certain commodity is 1000 units. At a unit price of $85, the demand drops to 600. What quantity would be demanded if the commodity was free? (I worked out the problem and came up with the equation -3/40×+130 but I don't know how to find the quantity that would be demanded if the commodity was free)

Answer 1

when the price drops from $85 to $55 ... $30
... the demand increases from 600 to 1000 ... 400 units

$55 to zero is a drop of $55

the change in demand is ... ($55 / $30) * 400 units

the change is added to the demand at the $55 price point