A parallelogram billboard has a base of 18 decimeters and a height of 15 decimeters. If the front and back sides are painted with paint, the total amount of paint is 2160 grams. How many grams of paint is used per square decimeter on average?
一块平行四边形广告牌，底是18分米，高是15分米，把它的正、反两面都刷上油漆，共用油漆2160克，平均每平方分米用油漆多少克？
<gadget id="calculator">18 * 15 * 2</gadget>
<output>540</output>

<gadget id="calculator">2_160 / 540</gadget>
<output>4</output>

<result>4</result>