Because the temperature drops 5 degrees every 1000 ft, we can say that

Δt = -5(x_{final}-x_{initial})/1000

where Δt is the change in temp, x_{final} is the final position and x_{initial }is the starting position.

Thus, we can get our answer by Δt = -5(15000 - 5000)/1000 = -5(10000)/1000 = -5(10) = -50 degrees.