How to determine if a probability problem has a binomial distribution or not? I'm studying for my Stats midterm and I am confused about the binomial distribution concept.
Among these 2 problems, why does the second question have a binomial distribution and not the first question?
1) In a large 2 lb bag of candies, 15% of the candies are green. The chances of pulling out at least one green candy in three tries is...
2) An owner suspects that only 5% of the customers buy a magazine and thinks that he might be able to use the display space to sell something more profitable. What is the probability that exactly 2 of the first 10 customers buy magazines?  
We already know about SPIN: S(Success/Failure) P(Probability) I(Independent) N(Fixed number of trials) and I'm curious as to why SPIN doesn't apply to the first problem as well?
Any help would be appreciated. Thank you.

Remember that the binomial distribution tells you what the odds are of getting exactly $x$ successes in $n$ attempts, given those attempts are bernoulli trials (Or they fit the restrictions of SPIN, as you say).
The first problem with question one is the part where it says "at least". That is not "exactly". This is more reminiscent of the CDF of the geometric distribution then a binomial distribution.
The second problem with question one is that the attempts are not independent. Taking out a bean changes all further tests. You can put this effect in by using the hypergeometric distribution, but this effect might be negligible if there are enough beans.
Still, you can get really close by using the binomial distribution for x = 1, 2, and 3.
