Question 328491
There a different ways to go about it, but I am going to convert everything to miles.

Since the plane is traveling 400 mph, in .6 minutes it travels x=4 miles.

We can build a right triangle. Let y=1/2 miles (2640 feet). This remains constant because the plane is not going higher or lower. Thus, dy/dt=0

dx/dt=400 mph

By Pythagoras, the distance from the observer to the plane is the hypoteneuse of the triangle: D={{{sqrt((1/2)^2+4^2)=sqrt(65)/2=4.031}}} miles.

So, we have {{{D^2=x^2+y^2}}}

Differentiate implicitly w.r.t time(t).

{{{D*(dD/dt)=x*(dx/dt)+y(dy/dt)}}}

Enter in the knowns and solve for dD/dt, the rate of change of the distance between the observer and the plane.

{{{sqrt(65)/2*(dD/dt)=4*(400)+(1/2)(0)}}}

{{{1600/(sqrt(65)/2)=640*sqrt(65)/13=396.91}}} mph