How to interpret Venn diagrams - SSAT Upper Level Quantitative

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Question

Sets

The above represents a Venn diagram. The universal set is the set of all positive integers.

Let represent the set of multiples of 7; let represent all of the multiples of 11; let represent all of the multiples of 13. As you can see, the three sets divide the universal set into eight regions. Suppose each positive integer was placed in the correct region. Which of the following numbers would be in the same region as 2,431?

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Answer

The region in which 2,431 appears depends on the sets of which 2,431 is an element, which in turn depends on which of 7, 11, and 13 divides it evenly:

2,431 is a multiple of 11 and 13, but not 7, so 2,431 is in and , but not . We look for a number among the choices that is in and , but not - that is, a number divisible by 11 and 13 but not 7.

, so 2,772 is divisible by 7. We can eliminate it.

, so 2,184 is divisible by 7. We can eliminate it.

, so 3,081 is not divisible by 11, and we can eliminate it.

, so 2,409 is not divisible by 13, and we can eliminate it.

However:

2,145 is divisible by 11 and 13 but not 7, so this is the correct choice.

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