The possible outcomes are {1, 2, 3, 4, 5, 6}. How many of these outcomes are multiples of 3? {3, 6}. So the probability of rolling a mult. of 3 is 2/6, or 1/3.
How many possible outcomes might be numbers less than 5: {1, 2, 3, 4 }. Thus, the prob. of rolling a number less than 5 is 4/6, or 2/3.
What is the prob. of rolling a mult. of 3 OR a number less than 5? These events are independent, so just take the 2 probabilities and add them together: 1/3 + 2/3 = 3/3 = 1. This event has a probability of 1.0.