The perimeter of a right-angled trapezoid is 30 cm, and the two sides are 4 cm and 6 cm respectively. What is the area of the trapezoid in square centimeters?
一个直角梯形的周长是30厘米，两条腰分别是4厘米和6厘米，这个梯形的面积是多少平方厘米．
<gadget id="calculator">30 - 4 - 6</gadget>
<output>20</output>

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

<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<result>40</result>