This sounds to me like a hypergeometric probability distribution problem. You have a finite population (100 accounts) and a known number of elements (25 delinquent accounts). You take a sample of 75 and you're trying to calculate the probability of getting all 25 elements in your sample of 75 from the finite 100. Please take a look at the formal calculation in a stat book or online. The probability equals [ (25 choose 25)*( (100 - 25) choose (75 - 25) ) ] / (100 choose 25). Hope that makes sense. Good luck!