document.write( "Question 885698: A plane flies 300 miles having a tail wind of 10 mph and returns against a tail wind of 20 mph. Find the speed of the plane in still air if the time of the flight is 4.5 hours. \n" ); document.write( "
Algebra.Com's Answer #535371 by lwsshak3(11628)\"\" \"About 
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A plane flies 300 miles having a tail wind of 10 mph and returns against a tail wind of 20 mph. Find the speed of the plane in still air if the time of the flight is 4.5 hours.
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\n" ); document.write( "let x=speed of plane in still air
\n" ); document.write( "(x+10)=speed of plane with tail wind
\n" ); document.write( "(x-20)=speed of plane against the wind
\n" ); document.write( "travel time=distance/speed
\n" ); document.write( "\"300%2F%28x%2B10%29%2B300%2F%28x-20%29=4.5\"
\n" ); document.write( "300(x-20)+300(x+10)=4.5(x-20)(x+10)
\n" ); document.write( "300x-6000+300x+3000=4.5(x^2-10x-200)
\n" ); document.write( "600x-3000=4.5x^2-45x-900
\n" ); document.write( "4.5x^2-645x+2100=0
\n" ); document.write( "solve for x by quadratic formula:
\n" ); document.write( "\"x+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+\"
\n" ); document.write( "a=4.5, b=-645, c=2100
\n" ); document.write( "ans:
\n" ); document.write( "x=3.33 (reject)
\n" ); document.write( "or
\n" ); document.write( "x=140
\n" ); document.write( "speed of plane in still air=140 mph\r
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