Answer: 
... 
against the wind, an airplane travels 3640 km in 7 hours. 
with the wind, the same plane travels 6000 km in 6hrs. 
... 
let speed of plane in still air be x mph 
speed of wind be y mph. 
.. 
t=d/r 
.. 
against wind speed = x-y mph 
with wond speed = x=y mph 
... 
3640/(x-y)=7 
3640 = 7x-7y 
 /7 
520=x-y.....................1
 
... 
6000/x+y = 6 
6000 = 6x+6y 
/6 
1000=x+y.....................2 
... 
Add (1) & (2) 
2x=1520 
/2 
x=760 mph speed of plane in still air 
... 
plug value of x in 2 
1000=760+y 
100-760=y 
240 = y 
240 mph speed of wind