Answer:
 r  = 225 Mil/h     speed of the airplane in still air
Step-by-step explanation:
Then:
d  is traveled distance   and r  the speed of the airplane in still air
so the first equation is for a 4 hours trip 
as  d = v*t
d  =  4 *  ( r  + 25)    (1)          the speed of tail wind  (25 mil/h)
Second equation the trip back in 5 hours
d  =  5  * ( r  - 25 )    (2)
So we got a system of two equation and two unknown variables  d  and
r 
We solve it by subtitution
from equation (1)     d  =  4r  + 100
plugging in equation 2
4r  + 100  = 5r -  125    ⇒   -r  =  -225     ⇒    r  = 225 Mil/h
And distance is :
d  =  4*r  +  100         ⇒ d  =  4 * ( 225)  +  100
d  =  900 +   100
d  = 1000 miles