Answer:
Speed of airplane in still air = 630 mile per hour
Speed of wind = 70 miles per hour.
Step-by-step explanation:
Let speed of plane in still air = 
 miles/hour
 miles/hour
Let speed of wind = 
 miles/hour
 miles/hour
Speed of airplane with the wind can be given by = 
 miles/hour
 miles/hour
Speed of airplane against the wind can be given by = 
 miles/hour
 miles/hour
Distance of the trip = 2800 miles
Time taken by airplane to travel with the wind = 4 hours
Speed of plane with wind = 

Distance of the return trip (same trip distance) = 2800 miles
Time taken by airplane to travel with the wind = 5 hours
Speed of plane with wind = 

So, we have the system of equations:
A) 

B) 

Using elimination method to solve.
Adding equation A to B to eliminate 
 .
.
 

+ 

We get 

Dividing both sides by 2.

∴ 

Using 
 in equation A to find
 in equation A to find 

 

Subtracting both sides by 630
 

∴ 
 
 
Speed of airplane in still air = 630 mile per hour
Speed of wind = 70 miles per hour.