The average number of yards gained by a college football team on a passing play is 8

Question:

The average number of yards gained by a college football team on a passing play is 8 - 12r, where r is the fraction of their total plays that are running plays. Their average gain per running play is 10 - 8r. What is their optimal fraction of running plays? At this value of r, what is the average gain per pass? The average gain per run