Question 618727
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Hi,
An airplane, with speed in calm air{{{highlight(r)}}} , flies 72 miles with a 30-mph tailwind and then flies back into the 30-mph wind...
round trip was 1 hour
Note D = rt OR D/r = t
  {{{72/(r+ 30mph) + 72/(r-30mph) = 1hr}}}  |Multiplying thru by (r+30)(r-30) so as all denominators = 1
   72(r-30) + 72(r+30) =  (r+30)(r-30) 
         144r = r^2 - 900
    r^2 - 144r -900 = 0
factoring
(r -150)(r+6) = 0
    r = 150mph   (throwing out the negative solution for speed)
   150mph was the airplane speed in calm air
CHECKING our Answer***
72/180 + 72/120 =  = 1