Question 23795
Let the speed of the wind be y
Let the speed of the plane be x

3(x+y)=900 ------------------> 3x+3y=900 (Equation 1)
5(x-y)=900 ------------------> 5x-5y=900 (Equation 2)
Multiply equation 1 by 5 and equation 2 by 3


15x+15y=4500 ----->Add
15x-15y=2700 ----->Add


30x=7200
x=240
Hence, the speed of the plane in still air is 240 mph.