She can run 18 miles in 3 hours. That means she could run 6 miles in one hour. So, if she can ride her bike 18 miles in 2 hours she can bike 9 miles per hour.
If she bikes for 27 (3 hours) miles and runs 9 miles (1.5 hours) then taking the total of miles dividing it by the time you would get Maya's speed.
