Sally is increasing her distance by 15 miles an hour while Ben is increasing his by 50 miles per hour.
Sally is 100 miles away at 5 hours so add 15 x 2 to 100 gives you 130 miles. This is how far away she is at 7 hours.
Ben is 250 miles away at 5 hours so add 50 x 2 gives you 350 miles. this is how far away he is at 7 hours.
350-130=220, Ben is the furtherst away by 220 miles.