The upper base of the trapezoid is 3 cm, and the height is 4 cm. It is known that its area is 20 square centimeters. How many centimeters is the lower base?
梯形的上底是3厘米，高是4厘米，已知它的面积是20平方厘米，下底是多少厘米？
<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">20 * (1/2)</gadget>
<output>10</output>

<gadget id="calculator">10 - 3</gadget>
<output>7</output>

<result>7</result>