The perimeter of a right-angled trapezoid is 24 cm, and the two sides are 4 cm and 5 cm respectively. What is the area of the trapezoid in square centimeters?
一个直角梯形的周长为24厘米，两条腰分别为4厘米和5厘米，梯形的面积为多少平方厘米．
<gadget id="calculator">24 - 4 - 5</gadget>
<output>15</output>

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

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<result>30</result>