Question 77740
720/(x-30)+720/(x+30)=10
(720[x+30]+720[x-30])/(x-30)(x+30)=10
10(x^2-900)=720x+21600+720x-21600
10x^2-9000=1440x
10x^2-1440x-9000=0
x^2-144x-900=0
(x-150)(x+6)=0
x-150=0
x=150 answer for ther plane's speed in still air.
proof
720/120+720/180=10
6+4=10