first find out how fast they run 1 mile by dividing the time they ram ( 30 minutes) by the number of miles they ran (4)
30 minutes / 4 miles = 7.5 minutes per mile
now we know how fast they run divide the new overall time (48 minutes) by the speed (7.5 minutes) to find the number of miles they will run
48 / 7.5 = 6.4 miles