Question

an athlete is running on a bike path with mile markers he is currently at mile marker 13 if his sister is 5 miles away determine her possible location

Answer 1

His sister is at mile marker 8,
Mile markers are a mile apart

Answer 2

I have absolutely no clue how mile markers work, but assuming they mark a single mile each, his sister will either be five miles in front, or five miles behind him.

The possible answers are mile markers 8 and 18.