Question 37237
Where did you get 12 and 20 from?
Since the entire time for the trip is 10h, it is going to be SPEED 1 + SPEED 2.
EQUATION:
{{{720/(x-30)+720/(x+30)=10}}}
720[(x+30)+(x-30)]=10[(x+30)(x-30)]
{{{720(2x)=10(x^2-900)}}}
{{{1440x=10x^2-9000}}}
{{{x^2-144x-900=0}}}
{{{x = (-(-144) +- sqrt( (-144)^2-4*1*(-900) ))/(2*1) }}} 
x=150
Hence, the speed of the plane is 150mph.