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(20 X 19)/2 = 190 Rationale: Each of the twenty unique objects can be paired with any other one of the objects to make 20 X 19 ordered pairs. However, each such pair will not be uniquely different from the pair with the same objects selected in opposite order. Therefore, 20 X 19 must be divided by 2.

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Q: How many unique pairs are in 20 unique objects?
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