Math Problem (Level 2) : Round trip of the mountain path (DD5)

Taro made a round trip to the mountain path in five hours.
In the flat way, walked at 3km/h, in the downhill paths walked 4km/h and in the ascent walked at 6km/h.
Find the distance of a round trip.





Answer
20 km

Solution
Average speed of 6km/h and 3km/h become 4km/h, it comes to that Taro walked for 5 hours at a speed of 4km/h. 
Therefore, the distance of the round trip is 4km/h × 5 hours = 20km.