recall your d = rt. distance = rate * time.
the person left at 8:00, driving at 40mph, it covered a distance of say d miles, then came back at 2:30, from 8am to 2:30 pm is 6½ hours or 13/2 hours, and it again covered the same d miles back.
if going took say t hours, then coming back must have taken the slack from 6½ and t, namely (13/2) - t.
