RandNegBinomial(successes, probability, number) 
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Generates number random negative binomial trials. RandNegBinomial returns the number of attempts before a successes successes where the probability of success on each attempt is probability.
For example, how many rolls of a die does it take to roll a total of four 6's?
You can simulate this problem thirty times using RandNegBinomial(4,1/6,30) and you might generate the following data.
19, 18, 23, 18, 14, 25, 52, 69, 17, 24, 21, 29, 12, 36, 52, 36, 17, 22, 8, 30, 8, 26, 11, 41, 18, 13, 41, 18, 30, 25
Note: If you type decimals using a comma (eg 3,2) you should enter this formula as RandNegBinomial(4; 1/6; 30)  using the ; as a separator
