A rectangular water tank is 40 cm long, 30 cm wide, and 35 cm deep. The original water depth is 10 cm. After putting a square iron block with a side length of 20 cm, the top surface of the iron block is still higher than the water surface. How many centimeters is the water surface at this time?
一个长方形水箱，从里面量长40厘米，宽30厘米，深35厘米．原来水深10厘米，放进一个棱长20厘米的正方形铁块后，铁块的顶面仍然高于水面，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>