cuboid water tank has a length of 40 cm, a width of 30 cm, and a depth of 35 cm measured from the inside. The height of the water in the tank is 10 cm. If a cube with a side length of 20 cm is placed in it, how many cm is the height of the water?
一个长方体的水箱，从里面量长40厘米，宽30厘米，深35厘米，箱中水面高10厘米，放进一块棱长是20厘米的正方体后，这时水面高多少厘米？
<gadget id="calculator">40 * 30</gadget>
<output>1_200</output>

<gadget id="calculator">1_200 * 10</gadget>
<output>12_000</output>

<gadget id="calculator">20 * 20</gadget>
<output>400</output>

<gadget id="calculator">1_200 - 400</gadget>
<output>800</output>

<gadget id="calculator">12_000 / 800</gadget>
<output>15</output>

<result>15</result>