There is a fish tank whose length, width, and height are 50cm, 30cm, and 40cm respectively. If there is 30L of water in it, how many centimeters is the water surface from the upper mouth?
有一个鱼缸长、宽、高分别为50cm、30cm、40cm，如果里面装30L水，水面离上口有多少厘米？
<gadget id="calculator">50 * 30</gadget>
<output>1_500</output>

<gadget id="calculator">1_000 / 1_500</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">30 * (2/3)</gadget>
<output>20</output>

<gadget id="calculator">40 - 20</gadget>
<output>20</output>

<result>20</result>