Two Birthday Problem

Why the probabilities are multiplied why not addition ?

for two person p = (365/365) * (364 / 365 )

why is it not addition ?
for two person p = (365/365) + (364 / 365 )

First, probabilities can’t add up to more than 1. So your addition example is mathematically invalid.

Multiplication is used if the events are independent, and you want to know if both of them happen.

What is the probability that I roll two dice and both of them are a six?
1/6 * 1/6 = 1/36.

You can also prove this example by writing out all of the possible combinations of two dice outcomes, and counting up how many combinations have six twice.

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This topic (multiplying or adding probabilities) is covered in several of the lectures.

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