trapezoidal billboard has an upper base of 6 meters, a lower base of 9 meters, and a height of 4 meters. If you want to paint this billboard, how much will it cost if you calculate it at 20 yuan per square meter?
一个梯形广告牌，它的上底是6米，下底是9米，高是4米．如果要给这个广告牌涂上油漆，按每平方米花费20元来计算，共要花多少元？
<gadget id="calculator">6 + 9</gadget>
<output>15</output>

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

<gadget id="calculator">15 * 2 * 20</gadget>
<output>600</output>

<result>600</result>