cuboid water tank is 80 cm long, 50 cm wide, and 30 cm high, and contains 80 liters of water. How many centimeters is the water surface from the top of the water tank?
一个长方体的水箱，长80厘米，宽50厘米，高30厘米，里面装有80升的水，水面离水箱上口多少厘米．
<gadget id="calculator">80 * 50</gadget>
<output>4_000</output>

<gadget id="calculator">1_000 / 4_000</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">80 * (1/4)</gadget>
<output>20</output>

<gadget id="calculator">30 - 20</gadget>
<output>10</output>

<result>10</result>