I am working the following probability problem which I came across online:
A computer network has 5 paths to its destination. Each path has a 20% chance of working and each path is independent. Am I guaranteed that at least one path will work?
I first answered it like:
P(1st path working OR 2nd path working OR 3rd path working OR 4th path
working OR 5th path working) = 0.2 + 0.2 + 0.2 + 0.2 + 0.2 = 1
but apparently that is not correct. Can someone explain why?
Thanks!
Best Answer
What you're doing has no right to work, because if there had been 6 paths each with 20% reliability, the same method would yield a probability of $1.2$ -- but probabilities cannot be more than $1$.
Your fundamental error is that you're adding independent probabilities. Doing so is never correct, because it amounts to ignoring the possibility that both independent events occur at the same time (which they can do because they're independent).
The smart way to do questions like this is to compute the probability that all paths will fail at the same time (which involves multiplication which is allowed for independent probabilities), and then invert that result.