What is the probability of rolling a number less than 5 on a six-sided die?

A way to visulize this is to start with asking "what situation has the least probability" on one roll of a six-sided die. The answer is any particular number, say a 1. Since there are six possible numbers, the probability is 1-in-6 or 1/6 = 0.17 (17%); now, since the total probability of any set added to the probability of the opposite outcome must be 1, then it stands that the probability of getting any of the 5 remaining numbers (2 through 6) is 1 - 0.17 = 0.83 (83%).

In fact, for any n-sided die, the probability, on one roll, of getting one of m numbers in the set of nnumbers is m/n.

Answer: if n = 6, and m = 4, because there are only 4 numbers on a six-sided die less than 5, the probability, P = 4/6 = 0.67 (67%).

Now let's extend the question and ask: what is the probability of getting a number less than 5 in each of two rolls. To answer, we must accept that each roll is a mutully exclusive set. That means the each roll has no impact whatsoeveron any other roll. Now, one could argue that this is never the case, because when the die bounces on the table, it changes the surface structure, or your hand muscles are stressed from the first roll, etc., but you would dramatically change the nature of the probability sets and make this far too technical a situation to be handled here. But in general, we say that these two die rolls have no impact on one another and are therefore mutually exclusive.

To find the probability of two mutually exclusive sets, we simply multiply the separate probabilities. Keep in mind that while the two sets are mutually exclusive, the sum of the combined probabilities, plus the opposite of the two, must be 1 (or 100%).

So, if the probability of rolling a 1,2,3, or 4 in one throw is 0.67, the doing the same in two throws is 0.67 * 0.67 = 0.44 (44%); hopefully you intuitively predicted that the probability would go down. Therefore, the probability of not throwing anything less than a 5 twice in a row is P = 1 - 0.44 = 0.56 (56%); keep in mind: this is not the probability of throwing a 5 or 6 twice in a row; it simply means not throwing a 1, 2, 3, or 4 twice in a row; for example, this could mean throwing a 1,2,3, or 4 the first time, but then throwing a 5 or 6 the second time. You have to ask "what are all the situations where it wouldn't be true?". Knowing that the probability of the opposite occurring is 56%, this should intuitively tell you that this "opposite" probability set has more possibilities.

Try this on your own: What is the probability of rolling exactly a 5, then rolling exactly a 1? Hint: what is the probability of each case on its own?