t1 = S / (3 * v1)
t2 = 2 * S / (3 * v2)
t = t1 + t2 = S / (3 * v1) + 2 * S / (3 * v2) = S * (2 * v1 + v2) / (3 * v1 * v2)
<v> = S / t = S * 3 * v1 * v2 / S * (2 * v1 + v2) = 3 * v1 * v2 / ( 2 * v1 + v2)
<v> = 3 * 120 * 60 / (2 * 120 + 60) = 3 * 120 * 60 / 300 = 72 км/ч
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t1 = S / (3 * v1)
t2 = 2 * S / (3 * v2)
t = t1 + t2 = S / (3 * v1) + 2 * S / (3 * v2) = S * (2 * v1 + v2) / (3 * v1 * v2)
<v> = S / t = S * 3 * v1 * v2 / S * (2 * v1 + v2) = 3 * v1 * v2 / ( 2 * v1 + v2)
<v> = 3 * 120 * 60 / (2 * 120 + 60) = 3 * 120 * 60 / 300 = 72 км/ч