We can define the events A and B for the die-tossing experiment: A: Observe an odd number B: Observe a number less than 4 Since event A occurs if the upper face is 1, 3, or 5, it is a collection of three sim- ple events, and we write A = {E, E3, Es). Similarly, the event B occurs if the upper face is 1, 2, or 3 and is defined as a collection or set of these three simple events: B={E, E2, E3}.