In a cuboid glass fish tank with a length of 50 cm and a width of 40 cm, a cube iron block with a side length of 10 cm is placed. The water depth in this fish tank is 20 cm. If the iron block is taken out, how many cm is the water depth in the tank?
在一个长50cm，宽40cm的长方体玻璃鱼缸中放入一个棱长10cm的正方体铁块，这个鱼缸内的水深20cm，如果拿出铁块，缸内的水深是多少cm？
<gadget id="calculator">50 * 40</gadget>
<output>2_000</output>

<gadget id="calculator">10 / 2_000</gadget>
<output>1/200 = around 0.005</output>

<gadget id="calculator">10 * 10 * (1/200)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">20 - (1/2)</gadget>
<output>39/2 = around 19.5</output>

<result>39/2 = around 19.5</result>