An old Chinese medicine doctor came to a remote mountain village.

The people here favor boys over girls. Seeing this situation, he felt he had found a way to get rich.
He put up a sign: "Boy-Only Formula, 10 taels per dose. If it doesn't work, the full price will be refunded, and an additional 2 taels compensation."
The villagers were skeptical at first, but gradually more and more people chose to believe.
After all, they wouldn't lose anything, so why not give it a try?
There are over 100 households in the entire village, almost every family bought it. The old Chinese medicine doctor earned 1,000 taels.
The probability of having a boy or a girl is almost equal, about 50%.
So, 500 taels are profit for the doctor, while the other 500 taels need to be refunded to villagers who had girls. Additionally, there is a 2-tael compensation, totaling 100 taels in compensation.
Therefore, in total, the old Chinese medicine doctor ultimately made a profit of 400 taels.
Strictly speaking, the doctor can still make money afterward because as long as some families have boys, other villagers will still buy the medicine with a mindset of trying.