Let E be a certain Event.
Let E' be the complement of that event, that is the denial or opposite, of that event.
Some examples:
E: Rolling a 2, from a die E': rolling 1, 3, 4, 5 or 6
E: Picking red E': picking any color except red
E: getting married in January E': getting married not in January.
Always, P(E)+P(E')=1
Thus if E: "raining", and P(E)=0.87
P(E')=P(not raining)=1-P(E)=1-0.87=0.13
Answer: 0.13
