15 km = 15000 meters
2 short run days = 8500*2 = 17000 m
2 long run days = 15000*2 = 30000 m
30000 - 17000= 13000 m
He will run 13000 meters farther on his two long run days.
You need to put the Answers and picture/problem before asking someone to answer it.
The property illustrated is the Commutative Property
