Observe the weather in a certain area in the next five days and remember ai as an event.

It doesn't rain for 0 ~ 6 days.

P(A 1)=P(A)

P(A2)=2P(A)

.

.

P(A5)=5P(A)

So the probability of no rain is: p (a1)+p (a2)+...+p (a5) =15p (a).

So the probability of rain is: 1- 15P(A).