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.
