College Math CLEP Prep Practice Exam

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What is the greatest common factor of 18 and 21?

  1. 1

  2. 3

  3. 6

  4. 9

The correct answer is: 3

The greatest common factor (GCF) of two numbers is the largest positive integer that divides both numbers without leaving a remainder. In this case, the GCF of 18 and 21 is 3, which means that 3 is the largest number that can evenly divide into both 18 and 21. Option A (1) is incorrect because 1 can divide into any number without leaving a remainder, not just into 18 and 21. Option C (6) and D (9) are not the GCF because they are not the largest numbers that can evenly divide into both 18 and 21. Therefore, the correct answer is B (3).

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