In spring of 2013, the cost for tuition at CCBC was $106 per credit for in-county residents. An in-county student, Joan, taking 12 billable hours paid a total of $1586 to the CCBC bursar. Another in-county student, Mark, taking 15 billable hours, paid a total of $______ to the CCBC bursar. Fill in the blank. Find the rate of change (slope) of cost per billable hour. Is the answer you received the value you expected? Explain.