![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/step1.JPG\")
Method 1:
\List the factors of each number.
\Factors of 20: 1, 2, 4, 5, 20, 20
\\
Factors of 21: 1, 3 , 7
\\
Factors of 25: 1, 5, 25
\There is no common prime factors.Three numbers always have 1 as a prime factor.
\so, the GCF is 1. \ \
\![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/step2.JPG\")
Method 2:
\Write the prime factorization of each number.
\20 = 2 × 2 × 5 .
\\
21 = 3 × 7.
\\
25 = 5 × 5 .
\There is no common prime factors.Three numbers always have 1 as a prime factor.
\so, the GCF is 1. \ \
![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/solution.JPG\")
There is no common prime factors.Three numbers always have 1 as a prime factor.
\so, the GCF is 1. \ \
\