The area of a trapezoid is 36 square centimeters, its upper base is 3 cm, and its height is 4 cm. What is its lower base in cm?
一个梯形的面积是36平方厘米，它的上底3厘米，高4厘米，它的下底是多少厘米．
<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">36 * (1/2)</gadget>
<output>18</output>

<gadget id="calculator">18 - 3</gadget>
<output>15</output>

<result>15</result>