The table below shows the relationship between the number of times you get your car washed each month and your total monthly benefit from car washes. Each car wash costs $15.
![54_benefit from car washes.png](https://secure.tutorsglobe.com/CMSImages/54_benefit from car washes.png)
What is the marginal benefit of the 2nd car wash each month? What is the optimal number of car washes per month? Why? Draw a graph to justify your responses.