The probability that a convenience store has no iced tea is 50%. If Karl is stopping by 3 convenience stores on his way to work, what is the probability that he will not be able to buy a can of iced tea?

what is the correct way of solving this?

--> (1/2)(1/2)(1/2) = 1/8

or

--> 1-(1/2)(1/2)(1/2) = 7/8

I often get confused when to use 1 - (probability of x not happening) vs just multiplying the probability that an even happens x times.

Anyone care to give their thoughts?

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Impossible is nothing