Average speed

The truck drove 1/2 of the way on the highway at 80km/h. The other half of the way 20km/h. Calculate the average speed

Result

v =  32 km/h

Solution:

v1=80 km/h v2=20 km/h  v=s/t=s/(s/2/v1+s/2/v2) v=2/(1/v1+1/v2)=2/(1/80+1/20)=32 km/hv_{1}=80 \ \text{km/h} \ \\ v_{2}=20 \ \text{km/h} \ \\ \ \\ v=s/t=s / (s/2/v_{1} + s/2/v_{2}) \ \\ v=2 / (1/v_{1} + 1/v_{2})=2 / (1/80 + 1/20)=32 \ \text{km/h}



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Duck10 Plays Yt
This is a Harmonic mean problem, 2ab/a+b will get you the answer.

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