The shepherd drove a flock of sheep across the 10 river. Every time we cross a river, half of the sheep fall into it. Every time he caught three, he finally found that there were six left. How many sheep were there before crossing the river?

Answer and analysis:

six

From the last six rivers and the ninth river, there is (6-3)2=6 (only). Similarly, it has been reversed to the original six.

The method of summarizing the reduction problem is backward deduction, which pushes forward from the result.