Xiaoyue used a 20cm long iron bar to form an isosceles trapezoid. Its height was 4cm and its waist length was 5cm. What is the area of this isosceles trapezoid in cm**2.
小悦用一根20cm长的铁条围成一个等腰梯形，量得它的高是4cm，一条腰长5cm．这个等腰梯形的面积是多少cm**2．
<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">20 - 10</gadget>
<output>10</output>

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

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

<result>20</result>