Question 119382
720/(r-30)+720/(r+30)=10
[720(r+30)+720(r-30)]/(r^2-900)=10
(720r+21,600+720r-21,600)/(r^2-900)=10 now cross multiply
10(r^2-9000)=1440r
10r^2-1440r-9000=0
10(r^2-144r-900)=0
10(x-150)(x+6)=0
x-150=0
x=150 for the speed of the airplane.
proof
720/(150-30)+720/(150+30)=10
720/120+720/180=10
6+4=10
10=10