If the speed of the plane is x,the speed of the wind is y,x+y=490,x-y=390,x=440,y=50.So the speed of the plane is 440 miles/h,the speed of the wind is 50 miles/h.
If a plane can travel 490 miles per hour with the wind and 390 miles per hour against the wind, find the speed of the plane in still air?