Question 26927
{{{720/(x+30)+720/(x-30)=10}}}
720[(x+30)+(x-30)]=10[(x+30)(x-30)]
{{{720(2x)=10(x^2-900)}}}
{{{1440x-21600=10x^2-900}}}
{{{10x^2-1440x-9000=0}}}
{{{x^2-144-900=0}}}
Factor: 
(x+6) and (x-150)
Remove the negative x=150


Hence, the speed of the plane in still ari is 150mph.
Paul.