Answer
$4$.
Work Step by Step
The given numbers are
$20,36$
Prime factorization of each number is
$20=5\cdot 2 \cdot 2$
$36=3\cdot 3 \cdot 2 \cdot 2$
Common prime factors are
$=2 \cdot 2$
Simplify.
$=4$
Hence, the greatest common factor is $4$.
