It will take 2 minutes for the cattle to come back.

It takes six minutes for a cow and a cow to cross the river.

It will take 2 minutes for the cattle to come back.

Cattle A and B finally crossed the river, which took 3 minutes.

A total of * * * takes 10 minutes.

Principle: The cow with the shortest time crosses the river first, so that other cows have a short waiting time.

Go to cattle A-B first. The waiting time of cattle C-D is 3 minutes that of cattle B-B, and then the waiting time of cattle C-D is 7 minutes, so the total time is * * *.

3+7 = 10 minutes

For the first time, the shepherd boy drove two cows, A and B, across the river, which took three minutes. Then ride back in armor, which takes 2 minutes.

The second time, the shepherd boy drove two cows, C and D, across the river, which took seven minutes. Then ride a cow back, 3 minutes.

The third time, it took 3 minutes to catch A and B across the river.

All four cows need to cross the river this time, just: 3+2+7+3+3 = 18 (minutes).

It is not very skillful to do this kind of topic, but mainly a matter of proficiency. Just do it more.