Learn in the textbookProbability and Counting (Number of Cases)
DefinitionA probability (確率) expresses how likely an event (something that happens) is, as a number from 0 to 1.
ExplanationWhen every outcome is equally likely, a probability is found as the number of cases in which the event occurs divided by the number of all possible cases. The probability that two events that do not affect each other (independent events) both occur is the product of their probabilities. Multiplying each possible value by its probability and adding the results gives the expected value, which shows the value you can expect on average.
AnalogyA 30% chance of rain in a weather forecast is simply the probability 0.3 written as a percentage.
Memory hookThe 率 in 確率 means a rate, so read a probability as a share with the whole as the denominator.