Question 247408
 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?
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Let s = speed required on the 2nd half to average 45 mph for the trip
Let d = total distance of the trip
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Write time equation; time = dist/speed
{{{.5d/40}}} + {{{.5d/s}}} = {{{d/45}}}
multiply equation by 360s to get rid of the denominator:
9s(.5d) + 360(.5d) = 8s(d)
:
4.5sd + 180d = 8sd
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Divide thru by d
4.5s + 180 = 8s
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180 = 8s - 4.5s
:
180 = 3.5s
s = {{{180/3.5}}}
s = 51.43 mph
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