Question 1091608
distance=speed*time
Luis will be caught t hours after he started walking, at a distance of 8t miles.
Kristin will catch Luis at a distance of 8x miles.  Her speed is 28 mph, so 28*(t-0.75)=8t, since her time is 3/4 of an hour less.
28t-21=8t
20t=21
t=21/20 hours , or 63 minutes;  Kristin started 45 minutes later, so it took her 18 minutes to catch him.
in that time, Luis will have walked 8 mph*1.05 hours=8.40 miles; Kristin will have ridden 28 mph*18 minutes or 0.3 hours, and that is 8.4 miles.