The taller boy would still win. The first race demonstrated that the taller boy could run 50 yards in the time it took the shorter boy to run 45 yards. So if the taller boy started five yards behind, they would draw level five yards from the end of the race, the taller boy having run 50 yards and the shorter 45 yards. As the taller boy is faster, he would run the final five yards more quickly, and win the race.