Question 71560
Use the d=rt formula.

t=d/r

You're given the time and distance.

{{{720/(r-30)+720/(r+30)=10}}}

{{{720(r+30)+720(r-30)=10(r+30)(r-30)}}}

{{{720r+21600+720r-21600=10r^2-9000}}}

{{{1440r=10r^2-9000}}}

{{{10r^2-1440r-9000=0}}}

Divide by 10:

{{{r^2-144r-900=0}}}

Factor or use the quadratic formula. Let's factor:

What 2 numbers when added equal -144 and when multiplied equal -900

a+b=-144
ab=-900

150 and -6

(r-150)(r+6)=0

Since speed can't be negative, -6 is erroneous.

Therefore, the planes speed in still air is 150 mph