Answer: = 240 mph.
Step-by-step explanation:
Let x = speed of the plane.
t= time taken
As per given, we have
[tex]t=\dfrac{450}{x+30}\ \ \ ...(i)\\\\ t=\dfrac{350}{x-30}\ \ \ ...(ii)[/tex] [Time = distance / speed]
From (i) and (ii), we get
[tex]\dfrac{450}{x+30}=\dfrac{350}{x-30}\\\\\Rightarrow\ 450(x-30)=350(x+30)\\\\\Rightarrow\ 450x-13500=350x+10500\\\\\Rightarrow\ 450x-350x=10500+13500\\\\\Rightarrow\ 100x=24000\\\\\Rightarrow\ x=240[/tex]
Hence, the speed of the plane = 240 mph.