Verified answer

v1=50 км/ч     v2=70 км/ч    vcp=?

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s/2=s1=s2

vcp=(s1+s2)/(t1+t2)

t1=s1/v1

t2=s2/v2

vcp=(s/2+s/2)/(s/(2*v1) + s/(2*v2))=2*v1*v2/(v1+v2)=2*50*70/(50+70)=58.3 км/ч

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