The Thoroughbred Bus Company Finds That Its Monthly Costs for One

Short Answer

The Thoroughbred Bus Company finds that its monthly costs for one particular year were given by $C ( t ) = 100 + t ^ { 2 }$ dollars after t months. After t months, the company had $P ( t ) = 1,000 + t ^ { 2 }$ passengers per month. How fast was its cost per passenger changing after 6 months

Enter your answer in dollars/month rounded to the nearest cent and without the units.