Question

The quantity demanded for a certain brand of portable CD players is 200 units when the unit price is set at $90. The quantity demanded for 1,200 units when the unit price is set at $40. Find the demand question.

Answer 1

Given that the quantity demanded for a certain brand of portable CD players is 200 units when the unit price is set at $90.

If we set quantity=y and price=x then in point form, we can write above information as (90,200)

Similarly "The quantity demanded for 1,200 units when the unit price is set at $40. " can be written as (40,1200)

Now we can plug both points into two point formula to find the required equation:

$\frac{y-y_1}{x-x_1}=\frac{y_2-y_1}{x_2-x_1}$

plug those points

$\frac{y-1200}{x-40}=\frac{200-1200}{90-40}$

$\frac{y-1200}{x-40}=\frac{-1000}{50}$

$\frac{y-1200}{x-40}=-20$

y-1200=-20(x-40)

y-1200=-20x+800

y=-20x+2000

Hence demand equation is y=-20x+2000