The upper base of the trapezoid is 5 decimeters, the lower base is 8 decimeters, and the height is twice that of the upper base. What is its area in square decimeters?
梯形的上底是5分米，下底是8分米，高是上底的2倍，它的面积多少平方分米．
<gadget id="calculator">5 + 8</gadget>
<output>13</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">(5/2) / 2</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">13 * (5/4)</gadget>
<output>65/4 = around 16.25</output>

<result>65/4 = around 16.25</result>