Other "solved problems" from early human history:

How can I find out if all the sheep have come in from the field? Count them as they are going out, and count them again as they are coming in.

How can I remember how many sheep that person owes me? Write down their name, the word "sheep", and the number of sheep.

How can I resolve conflicts with someone without fighting, when just talking it out with that person doesn't seem to be working? Find some person whom we both respect, have each of us explain their view on the situation, and follow the respected person's advice.