The method. Average speed is always calculated as follows: total distance ÷ total time, including stops. Time for each stage equals distance ÷ speed.

1. 1Total distance : 18 km uphill + 18 km downhill = 36 km.
2. 2Total time : ascent 18 ÷ 12 = 1.5 hr, break 0.5 hr, descent 18 ÷ 36 = 0.5 hr, giving 2.5 hr.
3. 3Average velocity : 36 ÷ 2.5 = 14.4 km/h.

The two pitfalls. 24 km/h averages 12 and 36, as if the ascent and descent took the same time; the ascent actually takes three times as long. 18 km/h divides 36 km by 2 hr, leaving out the break.

Key takeaways : divide total distance by total time, including stops.