I believe the formula for calculating probabilities for a geometric random variable P(x) = (1-p)^n-1*p is useless because if the students understood the problem, then the formula is not needed. It will only confuse the students and make it harder. So I developed this problem solving activity which has students develop their own way of calculating probabilities of a geometric random variable. The required prerequisite knowledge is calculating probabilities of compound independent events. My students also knew how to construct and use a tree diagram, but that isn't necessary. Only a few of my students used that method to calculate the probabilities. The lesson plan for this activity is to give them the activity and let them work in groups followed by a whole class discussion. There is no need for an intro, it explains everything in the document. It would even work if you have a sub.