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Solution to: Speedy Sums

Let the length of the journey be n km. The first half of the journey takes (½ × n) / 80 = 1⁄160 × n hours, and the second half of the journey takes (½ × n) / 120 = 1⁄240 × n hours. The average speed for the complete journey is n / (1⁄160 × n+1⁄240 × n) = 96 km/h.


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