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To find the greatest common factor (GCF) of an expression, we need to factor it completely.
The expression 20z - 4 can be factored by factoring out the greatest common factor of the coefficients and variables:
20z - 4 = 4(5z - 1)
Now, since 4 does not have any more factors, the GCF of 20z - 4 is 4.