All Probabilities Are Between 0 And 1

  • A probability of 1 means something is certain to happen
  • A probability of 0 means something definitely won’t happen
  • A probability of 0.5 means something is as likely to happen as it is not to happen

If All Results Are As Likely To Happen Then You Can Use This Formula

Probability = Number Of Ways For Something To Happen / Number Of Possible Results

Example: There are 8 balls in a bag. 3 are red. What is the probability of picking a red?

P(red) = 3/8 = 0.375

Probabilities Add Up To 1

So P(event happens) + P(event doesn’t happen) = 1

Example: A biased coin has a 0.7 chance of landing heads. What is the probability it will land tails?

P(tails) = 1 – P(heads) = 1 – 0.7 = 0.3