SOLUTION: 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/

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Question 397647: 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?
Answer by robertb(5830) About Me  (Show Source):
You can put this solution on YOUR website!
Let d = total distance
x = speed in the 2nd half of trip
==> length of time in 1st half of trip = %28d%2F2%29%2F40+=+d%2F80
length of time in 2nd half of trip = %28d%2F2%29%2Fx++=d%2F%282x%29
Then
d%2F%28d%2F80++%2B+d%2F%282x%29%29+=+45
1%2F%281%2F80++%2B+1%2F%282x%29%29+=+45
1%2F45+=+1%2F80++%2B+1%2F%282x%29
7%2F720+=+1%2F%282x%29 ==> x+=+360%2F7%0D%0A mi/hr.