Given:
Sandra Wallace walks at 5 miles per hour
Michael Sanders walks at 3 miles per hour
when Sandra has walked 20 miles
So, the number of hours = 20/5 = 4 hours
Michael starts to walk in a certain direction 2 hours before Sandra
So, the number of waking hours of Michael = 4 + 2 = 6 hours
So, the number of miles that Michael walked = 6 * 3 = 18 miles
So, the difference between them = 20 - 18 = 2 miles
So, the answer will be
Michael will be 2 miles behind Sandra