Statistics and Probability 7.SP.C.8.a
8a. Understand that, just as with simple events, the probability of a compound event is the fraction of outcomes in the sample space for which the compound event occurs.
Basically, we want students to know that we haven't thrown out everything we've learned about probability just because we're dealing with compound events now. All our same basic rules apply—defining the sample space, assigning probabilities, dividing the favorable outcomes by the total outcomes, and all that jazz.
Ah, but how will students know how to define the sample space? That's where 7.SP.8b comes in.