Question 64140
The plane travels 2*720 mi or 1440 mi in 10 hours 
d / t = r , so 1440 / 10 = 144 m i/hr is the planes 
speed in still air.
Whatever the windspeed is, it's effect in one direction 
will cancel it's effect in the opposite direction 
because the plane goes the same distance with
and against the wind. 
Actually, I believe this problem, as stated, is physically
impossible. The plane can't reverse direction without
slowing down and maintain it's speed in the opposite di-
rection.
Even if you change the problem and say that the plane keeps
going in the same direction for 1440 mi and it is only the
windspeed that reverses diretion at the halfway point,
any value that you select for the windspeed (other than 0)
will make the planes speed in still air turn out to be
greater than 144 mi/hr.
w = windspeed
p =planes speed instill air
The equation I end up with is
{{{p^2 -144p -w^2 = 0}}}
that's using
{{{720/(w+p) + 720/(-w+p) = 10}}}
The only way you can get p = 144 is if w = 0
If I say w = 18 mi/hr, I get p = 146.21 mi/hr
If I say w = 9 mi/hr, I get p = 144.5 mi/hr
There is a simple answer to this, but you just couldn't 
do it in real life. A lot of problems are like that
and my answer is- if a simple answer is what they 
want, give it to them, but watch out for the
"trick" questions when they want you to think a lot.
I could be all wrong about this, but I don't think so.