Answer:
a) When John is in the town (and suppose that he is travelling in a straigth line), he sees a 150 mi marker (means that he is 150 mi away from somewhere, let's suppose is a city), and latter, he sees a 112 mi marker (so he is now 112mi from the city) this means that he is closer to the city, and further to the town, then, the distance between john and the town will be 150 mi - 112 mi = 38 mi
b) currently, john is 38 mi ahead of the town and 112 mi away from the city.