To calculate probability we divide the number of favourable outcomes by the total number of possible outcomes. If we want to know the probability that it will rain, this will be the number of days in our database that it rained divided by the total number of similar days in our database. If we have data for 100 days with similar weather conditions (the sample space and therefore the denominator of our fraction), and on 60 of these days it rained (a favourable outcome), the probability of rain on the next similar day is 60100 or 60%.

Since a 50% probability means that an event has an equal chance of occurring and not occurring, 60%, which is greater than 50%, means that it is more likely to rain than not. But what is the probability that it won't rain? Remember that because the favourable outcomes represent all the possible ways that an event can occur, the total of the probabilities must be equal to 100%, so 100%−60%=40%, and so the probability that it will not rain is 40%.