## Elementary Algebra

$18$
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: $2 \cdot 3 \cdot 3 = 18$