Let X∼Negative Binomial(r,p)X \sim \text{Negative Binomial}(r, p)X∼Negative Binomial(r,p) where r=2,p=0.4r=2, p=0.4r=2,p=0.4. This counts the number of trials to get rrr successes. What is E[X]E[X]E[X]?
555
2.52.52.5
3.333.333.33
52\frac{5}{2}25