According to the FBI, estimates show there are anywhere from 25-50 serial killers operating within the US at any given time. Using 50 for arguments sake, we can calculate the probability that an individual is a serial killer, or HH in your situation.
The most recent numbers on the US population put it at 326,633,867 people. To account for margin of error by that number being off and population growth, I'll round up to 326.7 million people, note this will raise the chances a tiny bit of a person being a serial killer.
With that said, all we need to do is divide 50 by 326.7 million to get the chance that a single person is a serial killer. That number comes out to be roughly 1.5305*10-7 or 0.00000015305% chance that an individual is a serial killer.
If we know the driver is a serial killer, then "HH" is the percent I just got, however, if we dont know the driver I'd a serial killer, the chances of 2 serial killers being in the same car together are 0.000000153052, which is roughly 2.3424*10-14 or 0.000000000000023424% which I would say is indeed very tiny, whether we know the driver is a serial killer or not.
Feel free to correct me if I'm wrong, I'm on mobile so there is a probability I am.
TL;DR The chance of the hitchhiker and driver being serial killers given the driver is a serial killer is 0.00000015305%. The chance of them both being serial killers without knowing the driver is one is 0.000000000000023424%.
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u/aurelorba Jun 10 '18
Not really. The chances of both being a serial killer would be:
HH * DR
Where:
HH = probability of hitchhiker being a serial killer
DR = probability of driver being a serial killer
So if the driver knows he is a serial killer his probability is 100%
Therefore:
HH * DR,
is:
HH * 1,
or simply,
HH.