Question 414660

speed of the plane in still air = x mph,  speed of the wind = y mph

speed the the plane with wind i.e  x + y = 756/9 = 84


speed of the plane against the wind  i.e x-y = 756/18 = 42


now, after adding these equations,..

2x = 126

=> x = 63 

y = 84-63 = 21 


so speed of plane in still air = 63 mph

speed of the wind = 21 mph