Once I've covered basic probability and statistical measures, I like to have students work with conditional probability, which can often go hand-in-hand with tree diagrams. I created this worksheet for my Geometry students, and I find that it is challenging for them, but it can be broken down into "baby steps" and they learn to navigate their way around a tree diagram to calculate conditional probabilities.
I use this activity within cooperative groups, and I circulate to make sure they are ge