A cuboid container is known to be filled with water. For the first time, sink a small steel ball into the water; take out the small steel ball for the second time and sink it into a steel cone; take out the cone for the third time, sink another steel cylinder into the water, and the cylinder and the cone have the same bottom, etc. high. Now we know that the first spilled water is 6ml, and the second spilled water is 9ml, how many ml is the third spilled water.
已知一个长方体容器已注满了水．第一次把一个小钢球沉入水中；第二次取出小钢球沉入一个钢制圆锥；第三次取出圆锥，把另一个钢制圆柱沉入水中，并且这个圆柱和圆锥等底等高．现在知道，第一次溢出的水是6ml，第二次溢出的水是9ml，求第三次溢出的水是多少ml．
<gadget id="calculator">6 + 9</gadget>
<output>15</output>

<gadget id="calculator">15 * 3</gadget>
<output>45</output>

<gadget id="calculator">45 - 15</gadget>
<output>30</output>

<result>30</result>