Use the chance of no match to understand why shared birthdays arrive surprisingly soon.
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First lesson in this subject: Count equally likely outcomes
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With three people, there are three pairs that could match. With 23 people, there are 253 pairs. These overlapping possibilities are why a shared birthday can be likely even in a small group. Assume independent dates, each equally likely.
You are reading the opening. Members read every lesson and guide in full. A free account reads the first lesson in each subject in full.
You are reading the opening. Your free account reads the first lesson in each subject in full. Members read every lesson and guide in full.
First lesson in this subject: Count equally likely outcomes
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Practice · How small can a group be before matching birthdays become likely? Predict, generate groups and inspect the matches.
Book · Count outcomes, update chances with new information, and compare decisions through worked examples.
Previous lesson · Use complements and disjoint cases to compare multi-roll targets.
Next lesson · Separate a pattern’s chance in one window from the time you wait for it.
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