# Questions tagged [combinations]

Combinations are subsets of a given size of a given finite set. All questions for this tag have to directly involve combinations; if instead the question is about binomial coefficients, use that tag.

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### Birthday Problem: Finding a Probability Function of an Event

Problem: Ignoring leap days, the days of the year can be numbered $1$ to $365$. Assume that birthdays are equally likely to fall on any day of the year. Consider a group of $n$ people, of which you ...
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### Ways to choose $6$ numbers from $\{1, \dots, 20\}$ so that at least four are odd

How many ways are there to choose $6$ numbers from the set $\{1, \dots, 20\}$ so that at least four of them are odd? I came up with two different answers to this question, and I can't see why either ...
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### How do you calculate the possibilities of an alphanumeric string?

I want to know how many combinations are possible with a 4 character 4 digit string. All capital. Ex. ABCD1234, AAAA0000 - ZZZZ9999. What's the answer but more importantly, what's the formula? ...