Answer
$18$
Work Step by Step
Factor each number completely to obtain:
$36 = 2 \cdot 2 \cdot 3 \cdot 3
\\72 = 2 \cdot 2 \cdot 2 \cdot 3 \cdot 3
\\90= 2 \cdot 3 \cdot 3 \cdot 5$
Since one 2 and two 3s are common to both, the greatest common factor of the two given numbers is:
$2 \cdot 3 \cdot 3 = 18$
