Vivian's generous reply is very correct. I would just like to show you the way I would set up the solution. I like to use descriptive variable names. And I like to take lot's of paper space to setup these things. Even at the risk of looking repetitious. I encourage all my students to be work in this way. It either cuts down on dumb mistakes, or else it makes mistakes easier to track down.
Sp = still air Speed of the plane
Sw = the Speed of the wind
Sp + Sw = 480 ... in this case the wind speeds it up.
Sp - Sw = 400 ... in this case the wind slows it down.
We can solve for one of the unknowns, by adding the equations:
Sp + Sw = 480
+ (Sp - Sw = 400)
2Sp + 0 = 880
Sp = 880/2
Sp = 440
You can use this value in either equation, above, to find that the wind Speed, Sw, is 40.