Basics
Probability is a field that is a lot of fun, and is very important if you seek a career in commerce, management, programming, IT etc
Imagine a certain event - event "A". The probability of this event is P(A) = number of favourable outcomes divided by the total number of possible outcomes.
As you can see P(A) always lies between 0 and 1. P(A) in [0,1]
The probability of A given B is defined like this:
Also, remember that the probability of an event that is not A, is also very easy to calculate. Here's how:
P(A') = 1 - P(A)
The Spinner Problem
An RPG board game spinner has 4 equal sectors that are coloured yellow, blue, green and red. After you spin it, what is the probability of landing on each colour.
The answer is simple here, it's 1/4 for each side. However, let us look at the math to confirm.
P(A) = the number of ways to get A divided by the number of total outcomes.
So, there is only one way to get A (any of the 4 colours) and there is a total of 4 outcomes. Thus:
P(yellow) = 1/4, P(blue) = 1/4, P(green) = 1/4 and P(red) = 1/4. So, now you have proof that you have equal, 25% chance to get any of the colours.