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To derive the demand equation, we must understand that price is on the y axis while quantity is on the x axis.
Therefore, the slope of the demand curve is given by (75-100)/(80-60) = -1.25
The equation is Y=-1.25x + c
where Y - quantity demanded, x-price and c is a constant
takong one point say (80,75) we have 75 = -1.25(80)+c ;c= 175
Therefore demand equation is Y = 175-1.25X
The price above which there will be no demand is the price when quantity demanded is zero or the Y-intercept.
Hence, Y=175-1.25(0) =175
When price goes beyond 175, demand is zero.
The maximum quantity demanded is the quantity demanded when price is zero or the X-intercept.
Hence 0=175-1.25X; 1.25X=175
X=175/1.25 = 140
Maximum demand is 140
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