Editing Birthday problem (section)

=== Number of people until every birthday is achieved ===
The expected number of people needed until every birthday is achieved is called the [[Coupon collector's problem]]. It can be calculated by {{math|''nH<sub>n</sub>''}}, where {{math|''H<sub>n</sub>''}} is the {{mvar|n}}th [[harmonic number]]. For 365 possible dates (the birthday problem), the answer is 2365.