Answer:
The horizontal distance from the plane to the person on the runway is 20408.16 ft.
Step-by-step explanation:
Consider the figure below,
Where AB represent altitude of the plane is 4000 ft above the ground , C represents the runner.  The angle of elevation from the runway to the plane is 11.1°
BC is the horizontal distance from the plane to the person on the runway.
We have to find distance BC, 
Using trigonometric ratio,

Here,  ,Perpendicular AB = 4000
 ,Perpendicular AB = 4000 


Solving for BC, we get,

 (approx)
 (approx)
 (approx)
(approx)  
Thus, the horizontal distance from the plane to the person on the runway is 20408.16 ft