SOLUTION: Science and medicine. A plane flies 720 mi against a steady 30-mi/h headwind and then returns to the same point with the wind. If the entire trip takes 10 h, what is the plane’s

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Question 71560: Science and medicine. A plane flies 720 mi against a steady 30-mi/h headwind
and then returns to the same point with the wind. If the entire trip takes 10 h, what
is the plane’s speed in still air?
Answer by galactus(183) About Me  (Show Source):
You can put this solution on YOUR website!
Use the d=rt formula.
t=d/r
You're given the time and distance.
720%2F%28r-30%29%2B720%2F%28r%2B30%29=10
720%28r%2B30%29%2B720%28r-30%29=10%28r%2B30%29%28r-30%29
720r%2B21600%2B720r-21600=10r%5E2-9000
1440r=10r%5E2-9000
10r%5E2-1440r-9000=0
Divide by 10:
r%5E2-144r-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