The cross section of a canal is trapezoidal, the mouth of the canal is 30 decimeters wide, the canal bottom is 20 decimeters wide, and the canal is 10 decimeters deep. What is the cross-sectional area of this canal in square decimeters?
一条水渠的横截面是梯形，渠口宽30分米，渠底宽20分米，渠深10分米．这个水渠横截面面积是多少平方分米？
<gadget id="calculator">30 + 20</gadget>
<output>50</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">50 * 5</gadget>
<output>250</output>

<result>250</result>