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| 30 mph faster than the slow leg of the journey, therefore 90.
You know that speed(R) x time(T) = distance(D), and in this case the distance there (D1) and the distance back (D2) are the same, so
D1=D2
And, you know that R*T=D, so that
R1T1=R2T2
But, you're missing one piece of info to solve the problem. We're looking for average rate. We know that to find the average of something, we add the parts and divide by the number of parts, in this case we have two speeds. So, we take
R1+R2 and divide by 2
to give us the average rate for the entire round trip. We rearrange R1+R2 divided by 2 to give us
R1+R2 = 120.
You know that R1 is 30, so you solve for R2 to get you 90.
You can also figure it out by more quickly by realizing that the difference between the first speed and the average speed is 30 and in order to get the average speed to be 60 the second speed has to be be the greater than the ave speed by the same amount that the first speed is less than the average.
Edited by chris dds 3/17/2009 08:54
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