In august 2012, tropical storm isaac formed in the caribbean and was headed for the gulf of mexico. there was an initial probability of .69 that isaac would become a hurricane by the time it reached the gulf of mexico (national hurricane center website, august 21, 2012).
what was the probability that isaac would not become a hurricane but remain a tropical storm when it reached the gulf of mexico?

The answer is .31

Explanation:

The initial probability that tropical storm Isaac would become a hurricane was 0.69. We denote this event by A.

The probability that Isaac would not become a hurricane is the complement of event A, we denote the complement by Ac.

There are three probability axioms called Kolmogorov's axioms and from those axioms, we can deduce some useful rules for calculating probabilities.

This is one of them:

The probability of an event's complement (or the probability that the event will not happen) is 1 minus the probability that it will.

P(Ac)= 1- P(A)

P(Ac)= 1-0.69= 0.31