One way,
speed=50mph
distance=x miles
time=d/s=x/50 hours.....[1]
return trip,
speed=40mph
distance=x miles
time=d/s=x/40 hours.......[2]
Now we have been told that the total time taken is 7.2 hours
So add up equations [1] and [2],
therefore,he travelled 160 miles one-way
Hope this helps,
Prabhat