Answer :

If he walks 6/8 , he’ll walk farther.
Imagine the circle. Cut it up to 8 equal pieces in you head. If you did it, you almost get it. For now, let’s get back to fractions:
6/8 means take 6 piece of that circle. If you’ll divide 6 by 8, you’ll get ~0,7 (by other words ~70% of circle)
And 3/8 means take 3 pieces of 8 piece circle.
(3/8 ~ 0,3 ~ 30%)
Therefore 6/8 walks farther than 3/8