Students will learn how to represent counts of things using Venn diagrams. Then they'll learn the addition rule of probability, and how to use this rule to find the probability of mutually exclusive and non-mutually exclusive events. Here's a great introduction to the addition rule.
If you roll a die once, what is the probability of it landing 1 or 6? Here's the solution.
If you roll a die once, what is the probability of it landing on an even or prime number? Here's the solution.
Notes: That last MAA challenge is recommended for 8th graders, by the MAA. However, I disagree, as the idea of finding what's shared isn't done until learning how to count the intersection of things using Venn diagrams.