A local food bank uses volunteers to staff the kitchen. If there are 30 college students working there out of a total of 100 volunteers, what is the probability that in a sample of 10 volunteers, 4 of them are college students? Four decimal places please!

When you form a 10 people sample from a pool of large amount with equal probabilities, let s denote student and n denote non student, each spot in the sample has 1/2 chance to be s or n so each event has 1/2*1/2*1/2…. =(1/2)^10 probability. For example: snsnss. And intuitively you should see that there have a permutation 2^10 = 1024 ways to line them up (permutation with repetition like a suitcase 3-line passcode lock with 10 numbers on each line, you will have exactly 10*10*10 ways to choose passcodes, which is 10^3=1000 ways) , and each way has a probability a equal probability which is 1/1024 which is (1/2)^10.

Now let’s pick 4 students that we know for sure, mark 10 spots and have them walk randomly into each taking exactly 1 spot. There are C (10,4) possible combinations of choosing 4 people to walk in, for example 4s taking spot 3,6,7,10; 2,5,7,8; … C(10,4) = 210 ways. This represents that in a 10 spot line with 4 students and 6 non students in it, no matter how they are arranged there are 210 ways they line up. Now since each way has a prob 1/1024 we have a probability of 210 ways multiply by 1/1024 which = 55/256.

That was the case with large pool of equal number of S and N let’s make it a more realistic say 50 students and 50 non students. (1/2 derived from 50/100 or 5/10)

So now we have 30 students and 70 non students pool, the answer should be (3/10)^4*(7/10) ^6 which is the probability of 4 students and 6 non students lined up in some way while choosing them from the pool. Here we can also see that each 10 people sample no longer have the same probability. And that number came out to be 0.0081*0.117649=0.00095. And again there are 210 ways of arranging them so the probability is 210 ways multiply by 0.00095 which is 20%. This is the Answer.