You own a car dealership and you can now sell 50 cars per month at $28,000 per car and demand is decreasing by 2 cars per month each month. Each car costs the dealership $15,000 (wholesale cost). What is the fastest you can raise the price before your profit starts to drop?

For the revenue per month to drop, the price per car per month has to rise more than $1,500.

Explanation:

R = P*Q

dR/dt = (dP/dt) Q + P (dQ/dt)

dR/dt = (dP/dt) 40 + 20,000*3 > 0

(dP/dt) 40 > - 60,000

dP/dt > - 1,500

Therefore, For the revenue per month to drop, the price per car per month has to rise more than $1,500.