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:
Let p and w represent the speed of the plane and the speed of the wind, respectively. .. speed = distance/time .. p +w = 720/3 = 240 .. p -w = 720/4 = 180
Add the two equations to eliminate w. .. 2p = 420 .. p = 210 .. w = p -180 = 30
The speed of the wind is 30 mph. The speed of the plane in still air is 210 mph.