conical iron block is placed in a cylindrical container. Now turn on the tap and fill the container with water. In 2 minutes, the water just covered the top of the iron block; after another 3 minutes, the water just filled the container. It is known that the bottom area of the cylindrical container is 72 square centimeters, and its height is 21 centimeters; the height of the conical iron block is 9 cm, so what is the bottom area of the iron block?
一个圆柱体的容器内放有一个圆锥形铁块．现打开水龙头向容器内注水．2分钟时，水恰好没过铁块的顶点；再过了3分钟，水恰好注满容器．已知圆柱形容器的底面积为72平方厘米，它的高是21厘米；圆锥形铁块的高为9厘米，则铁块的底面积是多少？
<gadget id="calculator">72 * 21</gadget>
<output>1_512</output>

<gadget id="calculator">21 - 9</gadget>
<output>12</output>

<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<gadget id="calculator">72 * 4 * 5</gadget>
<output>1_440</output>

<gadget id="calculator">1_512 - 1_440</gadget>
<output>72</output>

<gadget id="calculator">3 / 9</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">72 * (1/3)</gadget>
<output>24</output>

<result>24</result>