8.4÷3=2.8

The speed of going up the mountain is 2.8 kilometers per hour.

8.4÷2=4.2

The downhill speed is 4.2 km/h.

Ask a math question: What is their average speed up and down the mountain?

8.4×2÷(3+2)=3.36

Their average speed up and down the mountain is 3.36 kilometers per hour.