Question 1086186
The planes travel at right angles to each other for 3 hours.  
The distance traveled by plane 1 after 3 hours is 120 mph* 3 h = 360 mi
The distance traveled by plane 2 after 3 hours is 140*3 = 420 mi
Their distance apart is the hypotenuse of a right triangle with legs 360 and 420.
Thus the distance is sqrt(360^2 + 420^2) = 553.2 miles