We basically want to know how many miles in real life 1 inch on the map represents. We already know that 4 inches on the map corresponds to 1.2 miles in real life, but we need to find what 1 inch on the map corresponds to. Let's make a proportion:
think about it this way. if the first group takes 17 hours, and the second group takes 4 hours more than the first group, then how long does the second group take?