Question 964017
A plane flies 720 mi against a steady 30-mi/h head-wind and then returns to the same point with the wind. If the entire trip takes 10 hr, what is the plane's speed in still air?
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let x=plane's speed in still air
(x+30)=plane's speed with the wind
(x-30)=plane's speed against the wind
travel time=distance/speed
{{{720/(x-30)+720/(x+30)=10}}}
lcd:(x+30)(x+30)=(x^2-900)
720x+21600+720x-21600=10(x^2-900)
1440x=10x^2-9000
10x^2-1440x-9000=0
x^2-144x-900=0
(x-150)(x+6)=0
x=150
what is the plane's speed in still air? 150 mph