Given that the average speed of going up and down the mountain is 4.8km/h, the total time of going up and down the mountain is (24x2) ÷ 4.8 = 10h.

The time to climb the mountain is 24÷4=6 hours.

Therefore, the downhill time is 10-6=4 hours.

So the downhill speed is 24 ÷ 4 = 6 km/h.

That is, the man walked down the mountain at a speed of 6 kilometers per hour.

I hope my answer is helpful to you.