document.write( "Question 1137736: On a journey of 200 km, a motorist found that if he were to increase his average speed by 2 km/h, the journey would take 15 minutes less. Calculate his average speed \n" ); document.write( "

Algebra.Com's Answer #755608 by ikleyn(52782) $\"\"$ $\"About$

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document.write( "Let x be his average speed (in km/h).\r\n" );
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document.write( "Then from the condition you have this  \"time\"  equation\r\n" );
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document.write( "     -  = 0.25   of an hour   (0.25 = 0.25 hour = 15 minutes)\r\n" );
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document.write( "To solve it, first multiply both sides by  4x*(x+2).  You will get\r\n" );
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document.write( "    800*(x+2) - 800x = x*(x+2)\r\n" );
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document.write( "    1600 = x*(x+2)\r\n" );
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document.write( "    x^2 + 2x - 1600 = 0\r\n" );
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document.write( "     =  =  = .\r\n" );
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document.write( "Of the two roots, only positive root x=  = 39.0125 is meaningful.\r\n" );
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document.write( "Answer.  His average speed is 39.0125 km/h  (approximately).\r\n" );
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document.write( "CHECK.    -  = 0.25.    ! Correct !\r\n" );
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