The distance between A and B is 48 kilometers. One part of the road is uphill and the rest is downhill.

Then the return journey is just the opposite, the downhill part becomes uphill and the uphill part becomes downhill.

Then: a round trip is equivalent to * * * uphill 48KM and downhill 48 km.

You can draw a picture to make it clearer.

So:

48÷8+48÷ downhill average speed =2.5+4.8.

48÷ downhill average speed =2.5+4.8-48÷8

Average downhill speed = 48(2.5+4.8-48÷8)

=48÷ 1.3=480/ 13 =36 12/ 13

About 37 kilometers per hour

Students in the fourth grade should be able to understand the problem in this way.