# What is Probability

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Relative frequency is different from actual probability. If we perform the event more and more times, then the relative frequency approaches to actual or true probability.
Relative frequency is the ratio of number of observed frequency outcomes to the total frequency of that experiment.
Example: Suppose an experiment is done for N times and the number of observed outcomes is p then relative frequency is p/N.

Theoretical probability is the ratio of the outcome we expect to the total number of times the experiment repeated.

Probability1 is given to the event which occurs. We will work with positive probabilities. Probability of at most 1 sibling is the sum of probability of zero sibling and probability of one sibling.
• Odds of an event = (Event not occurs)(Event occurs)
• Probability (not an event) = 1 – probability (event)
• npr = n!((n-r)!)
• ncr = n!(r!(n-r)!)
• ncr = ncn-r