minji391 wrote:

I see that is the answer, but what is wrong with adding two seperate speed and divide into two?

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Adding to pushpitkc explanation:

Hi

minji391,

annusngh Since time duration for both the speed is different so you can't simply add and divide it by 2.

Let me explain with some values.

For simplicity consider x = 5hrs and y = 10 hrs.

Speed during 1st half (S1) = 50/5 = 10 miles/hr

Speed during 2nd half (S2) = 50/10 = 5 miles/hr

~~Average speed = (S1+S2)/2 = (10 + 5)/2 = 7.5~~,

which is wrong.Actual average speed = 100/(10+5) = 6.66 miles/hr

Please note that time duration is different for each speed. So, you need to apply the weighted average formula.

Weighted average = (5*10 + 10*5)/(5+10) = 6.66 miles/hr.

Read following post for more detail:

weighted-averagesHope it helps.