# Chapter 2 - Real Numbers - Chapters 1-2 Cumulative Review Problem Set - Page 91: 26

9

#### Work Step by Step

First, we express each number as a product of its prime factors: $63=3\times3\times7$ $81=3\times3\times3\times3$ Since two 3s are common to both of the numbers, we obtain: Greatest Common Factor$=3\times3=9$

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