Question 1016840
{{{x}}}= speed of the plane in still air, in mph;
{{{y}}}= wind speed, in mph.
Then,
{{{x+y=480}}}= speed of the plane with the wind, in mph, and
{{{x-y=400}}}= speed of the plane against the wind, in mph.

Adding up both equations, we get
{{{x+y+x-y=480+400}}}
{{{x+x=880}}}
{{{2x=880}}}
{{{x=880/2}}}
{{{highlight(x=440)}}}