Answer:
For the revenue per month to drop, the price per car per month has to rise more than $1,500.
Step-by-step 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.