document.write( "Question 247408: Donna drove half the distance of a trip at 40 mi/h. At what speed would she have to drive for the rest of the distance so that the average speed for the entire trip would be 45 mi/h? \n" ); document.write( "
Algebra.Com's Answer #180502 by oberobic(2304)\"\" \"About 
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We know that:
\n" ); document.write( "d = rt, where d = distance, r = rate, t = time.
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\n" ); document.write( "We are told she drove 1/2d at r=40. We are not told how long it took.
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\n" ); document.write( "We want to find how fast she needs to drive for her average speed to be 45.
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\n" ); document.write( "The obvious answer is 50, since (40+50)/2 = 45.
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\n" ); document.write( "In other words, she drove 1/2d * 40 + 1/2d * 50 = 20 + 25 = 45.
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\n" ); document.write( "The problem is that time has been ignored with this solution.
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\n" ); document.write( "Assume the distance to be traveled is 80 miles.
\n" ); document.write( "At 40 mph, the first half of the trip will take 1 hr @ 40 mph.
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\n" ); document.write( "To average 45 mph for the whole 80 miles, the total time is:
\n" ); document.write( "80 = 45t
\n" ); document.write( "45t = 80
\n" ); document.write( "t = 80/45 = 16/9 = 1 + 7/9 hrs.
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\n" ); document.write( "Since the first hour is gone, the remaining distance must be traveled in 7/9 hr.
\n" ); document.write( "40 = r * t = r * 7/9
\n" ); document.write( "360 = 7r
\n" ); document.write( "7r = 360
\n" ); document.write( "r = 51.428
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