The area of a trapezoid is 56 square centimeters, its lower base is 18 cm, and its height is 4 cm. How many centimeters is its upper base?
一个梯形的面积是56平方厘米，它的下底是18厘米，高是4厘米，它的上底是多少厘米？
<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">56 * (1/2)</gadget>
<output>28</output>

<gadget id="calculator">28 - 18</gadget>
<output>10</output>

<result>10</result>