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Solution to: Speedy Sums

Let the length of the journey be n km. The first half of the journey takes (1/2×n)/80 = 1/160×n hours, and the second half of the journey takes (1/2×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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