Susie can run 2.5 miles per hour faster than jeff. in the time that it takes susie to run 6 miles, jeff runs 4 miles. find the speed of each runner.

So  if Susie can run 2.5 miles per hour more, 
so 
susie                                                Jeff
+2.5 per hour                                       0.0
susie runs 6 miles                               jeff runs 4 miles

8.5 miles per hour                                4 miles per hour