Basic College Mathematics (9th Edition)

$180$.
$12 = 2 \cdot 2 \cdot 3$ $15 = 3 \cdot 5$ $18 = 2 \cdot 3 \cdot 3$ $20 = 2 \cdot 2 \cdot 5$ LCM $= 2 \cdot 2 \cdot 3 \cdot 3 \cdot 5 = 180$.