A children's amusement park plans to build a new "candy house". It occupies a trapezoidal shape with an area of 720 square meters. It is known that the lower base is 30 meters and the height is 18 meters. How many meters is the upper base of this trapezoid?
某儿童游乐园打算新建一个"糖果屋"．它的占地面是一个梯形状，面积为720平方米．已知下底是30米，高是18米，这梯形的上底是多少米？
<gadget id="calculator">2 / 18</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">720 * (1/9)</gadget>
<output>80</output>

<gadget id="calculator">80 - 30</gadget>
<output>50</output>

<result>50</result>