So 1 1 began to go downhill, and 1 hour previewed at the top of the mountain.

So he must reach the mountain at 10.

Two hours down the mountain, Xiao Ming walked 2km more than up the mountain, and walked more than the total distance of the mountain 1km.

Xiaoming 1 hour fast downhill journey is 2km less than normal downhill journey.

So Xiaoming's downhill speed is (1+2) ÷ (2-1) = 3 km/h.

Climbing speed = 3-1= 2km/h

So the distance up the mountain is 3* 1+2=5km.

The time required to climb the mountain is 5 ÷ 2 = 2.5h.

To reach the top of the mountain at 10, you need 10-2.5=7.5.

That is, we leave home at 7: 30.