A hiker covers three equal stretches of a trail: uphill at 10 km/h, on the flat at 20 km/h and downhill at 60 km/h. What is her average speed for the whole trail?

Explanation

1. 01

   Let each stretch be 60 km (any convenient length works).

2. 02

   Times: $\frac{60}{10} + \frac{60}{20} + \frac{60}{60} = 6 + 3 + 1 = 10$ h.

3. 03

   Average speed $= \frac{180}{10} = 18$ km/h.