Idiot\'s Guides Basic Math and Pre-Algebra

Chapter 18: Probability 259

The Least You Need to Know

• To count quickly, take the number of options for each decision and multiply.


• The probability of an event is the number of ways it can happen over the total
  number of things that can happen.


• The probability of A and B is the probability of A times the probability of B.


• The probability of A or B is the probability of A plus the probability of B minus
  the probability of both.