According to the hair doctor: Baldness is caused by "bad hair".
Fact: "bad hair" kills "good hair", 1 "bad hair" can kill 100 "good hair".
Fact: An average person has an average of 100,000 hairs, out of which 0.1% are "bad hair", 99.9% are "good hair".

A hair machine can identify a bad hair and terminates it before it kills the good hairs.

We have 2 brands of hair machines:
Brand A: It identifies 50% of bad hair as bad hair, and will also misidentify 1% of good hair as bad hair.
Brand B: It identifies 90% of bad hair as bad hair, but will also misidentify 10% of good hair as bad hair.

Question: which Brand of hair machine will you buy? How many hair will you end up with that hair machine?

Note: In a 1978 survey, 85% of doctors got the wrong answer
This is a classic "Bayesian Theorem" dilemma (but I switched the "mammography" with the "hair machine")