Answer
$4$.
Work Step by Step
The given numbers are
$28,64$
Prime factorization of each number is
$28=2\cdot 2 \cdot 7 $
$84=2\cdot 2 \cdot2\cdot2 \cdot2\cdot2$
Common prime factors are
$=2\cdot 2 $
Simplify.
$=4$
Hence, the greatest common factor is $4$.
