SOLUTION: If someone runs the 1st half of a race at 5mph and the 2nd half at 10mph then what's the average speed?

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Question 541453: If someone runs the 1st half of a race at 5mph and the 2nd half at 10mph then what's the average speed?
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
You can put this solution on YOUR website!
If someone runs the 1st half of a race at 5mph and the 2nd half at 10mph then what's the average speed?
:
let d = the halfway distance of the race
let a = average speed for the whole race
:
Write a time equation, time = dist/speed
:
time at 5mph + time at 10mph = total time
d%2F5 + d%2F10 = %282d%29%2Fa
multiply by 10a, results
2ad + ad = 10(2d)
3ad = 20d
divide both sides by d
3a = 20
a = 20/3
a = 62%2F3 mph