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Probability

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Alice rolls a fair six sided die, and Bob rolls a fair twenty sided die.  What is the probability that the product of the numbers they roll is a multiple of 5?

Jul 27, 2022

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There are $$6 \times 20 = 120$$ total outcomes.

Of these, $$5 \times 16 = 80$$ outcomes have neither Alice nor Bob rolling a multiple of 5

So, there are $$120 - 80 = 40$$ successes, so the probability is $${40 \over 120} = \color{brown}\boxed{1 \over 3}$$

Jul 27, 2022