I recently read a question. It was: If 100 birds are sitting in a circle and all of them peck a bird either on their left or on their right(adsbygoogle = window.adsbygoogle || []).push({}); randomly,what's the expected number of birds that will be unpecked? The answer to this is 25 birds as probability of not being pecked is 25%. The logic went like this: The probability of a bird not getting pecked from right side will be 50% and same with the left too and then we have to halve this to get the answer. But why did we have to halve to get the probability, I mean is there any formula or axiom for this that it's how probability is treated? I know it's a very basic question and we can all solve such problems without even thinking about it, but what is the reasoning behind this that when we considered 2 sides instead of 1 we had to divide the probability by 2?

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# B Calculation of probability

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