Explanation: I believe it would be 16 miles, if he starts off at 6 miles on day one and goes up 1/2 mile every day, he would be up a mile every other day, basically on day 3 he would be at 7 and day 5 at 8 miles, if the pattern continues he would be at 16 miles by day 21.
Because the oppsite lines are parallel (you can tell because of the arrows) the opposite lines are the same length and the opposite angles are the same size.