charlieh65 wrote:This may be a stupid question, but I'm driving myself crazy trying to figure out what I'm doing wrong so hopefully you can help!

Alison has a 1/4 probability of going to college. Brandon has a 1/2 probability of going to college. Charlie has a 1/4 probability of going to college. What is the probability that Alison, Brandon, or Charlie will go to college?


I keep thinking back to..."AND means MULTIPLY, OR means ADD." But of course there isn't a 100% chance that one of them is going to college. help!

Just so you have it clear, or means add when the probabilities we are talking about are parts of a set of related mutually exclusive events. For instance, if the question asked "What is the probability of Alison going to college or not going to college?" then you could add the two probabilities and get 100%. The two events are mutually exclusive parts of the set of Alison's possible college decisions.

If you were drawing a card from a deck and wanted to know the chance of getting the king of spades or the jack of spades, you could add the two, 1/52 + 1/52, and get 2/52, which makes sense when you think about it. Each probability is an element of the set of probabilities of mutually exclusive results of drawing one card from the deck. All the probabilities in that set add up to 100 percent. If within that set there are multiple events considered favorable, you can just add their probabilities to determine the probability of a favorable event occurring.

On the other hand, if you were asking "In drawing one card from each of two decks what is the chance of getting at least one of the following, the jack of spades from the first deck or the king of spades from the other?" you can't just add their probabilities. The two events are not mutually exclusive elements in the same set of probabilities.