Probability Models. Cereal.
On average, I expect to have to check just over 11 emails before I find a real message. There’s slightly less than a 7% chance that my first real message will be on the 4th check.
In groups of 20 randomly selected blood donors, I expect to find an average of 1.2 universal donors with a standard deviation of 1.06. About 31% of the time, I would find 2 or 3 universal donors among 20 people.