Laura completed a 10-mile race in 90 minutes. She ran most of it, but she also walked a little bit. She knows that she usually runs at a pace of 8.5 minutes per mile and she usually walks at a pace
of 12 minutes per mile. Assuming that she kept these times during the periods of running and
walking during her 10-mile race, how many minutes did she spend running and how many minutes
did she spend walking? Write a system of equations, but do not solve.