How to quickly choose the right one with a 50% probability?

The probability of 50% is relatively high. According to the upper limit of the winning rate of 100%, if you buy lottery tickets, if the winning rate is 50%, you have a 50% chance of winning the grand prize, which is very high to a certain extent, not just half and half.

50% is only theoretical. No coin can balance the weight of both sides. If you can bring your own coin, find a dirty old coin, clean one side and try it. Find more similar coins and find out the rules. Even if you use the dealer's coins, you can check the wear and tear of the old and new coins. Of course, since it is gambling, technology is only one aspect, and the key is luck.