What is the formula for calculating probability?

Answer 1

short answer; it depends.

Generally, you can say the probablility of an event having a specific outcome is the number of ways in which that outcome can occur divided by the total number of ways any outcome can occur. For example, the likelihood of drawing a heart from a standard 52 card deck is 13/52 because there are 13 possible results that have the desired outcome (IE 13 heart cards) and 52 total possible outcomes (52 possible cards).