Originally Posted by

**spengel**
This sounds to me like a binomial probability distribution problem. Your calculation is a hypergeometric equation. But you don't really know the finite quantity of the population. So, you could consider 40% as the success rate and 60% as the failure rate in a binomial problem. The probability that 3 students did not study means that 2 students did study out of the 5 students sampled.

So, using the binomial formula: (5 C 2)*(0.4)^2 *(0.6)^3 = 0.346

The probability is an approximation that is pretty good as long as the sample size is no more than 5% of the total population. Anyway, that's what I'm thinking. Hope this helps!