Maths Olympiad Prep

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Combinatorics Difficulty 3.2 AMC 10/12 Find the answer

Two dice are thrown. What is the probability that the product of the two numbers is a multiple of 5?

Pick one

Solution

This is equivalent to asking for the probability that at least one of the numbers is a multiple of 55, since if one of the numbers is a multiple of 55, then the product with it and another integer is also a multiple of 55, and if a number is a multiple of 55, then since 55 is prime, one of the factors must also have a factor of 55, and 55 is the only multiple of 55 on a die, so one of the numbers rolled must be a 55. To find the probability of rolling at least one 55, we can find the probability of not rolling a 55 and subtract that from 11, since you either roll a 55 or not roll a 55. The probability of not rolling a 55 on either dice is (56)(56)=2536\left(\frac{5}{6} \right) \left(\frac{5}{6} \right)=\frac{25}{36}. Therefore, the probability of rolling at least one five, and thus rolling two numbers whose product is a multiple of 55, is 12536=1136,D1-\frac{25}{36}=\frac{11}{36}, \boxed{\text{D}}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.