To find the greatest common factor (GCF) of 20 and 16, we can use the prime factorization method.
First, we factor each number into its prime factors:
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The prime factorization of 20 is: \[ 20 = 2^2 \times 5 \]
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The prime factorization of 16 is: \[ 16 = 2^4 \]
Next, we identify the common prime factors and their lowest powers in both factorizations:
- The only common prime factor is \(2\).
- The lowest power of \(2\) in the factorizations is \(2^2\).
Therefore, the GCF is: \[ 2^2 = 4 \]
Thus, the greatest common factor of 20 and 16 is \( \boxed{4} \).