I have just satarted a degree in Computer science, and i was wondering if you can explain set theory to me. I will really appreciate it.

Well I'm confused exactly what with set theory you need help with. It's a very broad topic. Is this for a discrete math course?

Basically, a set consists of elements that can represent anything. You could have the set of letters in the alphabet, a set of 10 random numbers, etc. Sets are the foundation of probability.

For example, a classic question is, what is the probability of rolling a 2 on a fair die? The sample set is the set of all numbers that possibly could be rolled ... S = {1, 2, 3, 4, 5, 6}. The number 2 only appears once in the sample set, meaning that the probability of rolling a 2 is 1 out of 6 (1/6th). Now what is the probability of rolling a number greater than 4 on a fair die? Out of the sample set, this can be either a 5 or a 6. Therefore, the answer is 2 out of 6 (1/3rd).

if the set of tv stations you receive is {2,4,5,7,9,13}, what's the probability that ALL of those stations are showing a commercial at any given time?

(hint: 9 is a pbs station, so they're doing a pledge break)

Zero, because channel 9 never shows commercials, so it's impossible for all stations to be showing a commercial at the same time.