Probability is the likelihood of an event happening. Some other synonyms for probability are chance and odds. When you say, "I'll probably be at school tomorrow," you are saying it is likely you will be at school tomorrow, that there is a good chance you will be at school tomorrow.
The notation P(A) is used to show probability. To write that the probability of rain is 50%, write P(rain)=0.5. Notice that a decimal number is used, not a percentage.
In probability, we talk about or use:
When one flips a coin and records the result, one is doing an experiment in probability. The parts of the experiment are:

When one flips both coins and records the result, one is doing an experiment in probability. The parts of the experiment are:
What if one does not care which coin comes up heads and which coin comes up tails? Then a tails then a heads (TH) is the same as a heads then a tails (HT). This means that there are 3 outcomes: HH, HT, TT. The parts of the experiment then are:

Now take a look at rolling two dice. Usually one does not care which die has which value. One only cares about the sum of the dice.

Four basic principles of probability are:
#  A  B  C  D 
E  F  G  H  I 
J  K  L  M  N 
O  P  Q  R  S 
T  U  V  W  X 
Y  Z 
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