For a trapezoidal vegetable plot, the upper base is 9dm, the lower base is 12dm, and the height is 18dm. If each vegetable occupies an average area of 9d㎡, how many vegetables can be grown in this vegetable plot?
一块梯形的菜地，上底是9dm，下底是12dm，高是18dm，如果平均每颗菜占地9d㎡，这块菜地一共可以种菜多少颗？
<gadget id="calculator">9 + 12</gadget>
<output>21</output>

<gadget id="calculator">18 / 2 / 9</gadget>
<output>1</output>

<gadget id="calculator">21 * 1</gadget>
<output>21</output>

<result>21</result>