Question 830352

Please could you help me with this question: 
"Sarah goes up a hill and then down again the same way.On the way up she averages 2 miles per hour. On the way down she averages 4 miles per hour. What is Sarah's average speed in miles per hour for the whole journey?"
I know that the answer is 2.6 miles per hour but I'm not sure how to work it out.
Thank You<pre>
Average speed = total distance ÷ total time

Let distance, one-way be D
Total distance:                 {{{2D}}}
Time uphill:                    {{{D/2}}}
Time downhill:                  {{{D/4}}}
Total time for entire journey:  {{{D/2 + D/4}}}, or {{{(2D + D)/4}}}, or {{{3D/4}}}

Average speed = {{{2D}}} ÷ {{{3D/4}}}, or {{{2D(4/3D)}}}, or {{{8/3}}}, or {{{highlight_green(2&2/3)}}} mph
You can do the check!! 

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