David Chang, a student, mows lawns part-time. He determined that the average lawn mowing charge in his neighborhood is $30 per lawn, and he set his price to match. Soon, he had enough demand to fill his self-imposed quota of 12 lawns. As and when existing customers decided to mow their own lawns or hired another lawn mowing service, David would seek new engagements. Over time, David notices that he is spending more and more time mowing lawns. Whereas he was devoting 15 hours a week when he started, he finds himself putting in more than 20 hours during recent times. This trend puzzles him as he is still mowing only 12 lawns per week and the average rate in the neighborhood is still $30 per lawn.

Required:
Help David understand the root causes for the increase in the time required to mow 12 lawns.