In a cuboid container with a square bottom surface filled with water, put a cuboid iron rod vertically. The iron rod is completely immersed in the water. The height of the water surface in the container rises from 60cm to 75cm. It is known that the side length of the bottom surface of the container is 40cm. The length of the bottom side of the iron rod is 20cm, find the length of the iron rod.
在一个盛有水的底面是正方形的长方体容器中，垂直放入一根长方体铁棒，铁棒完全浸入水中，容器中的水面高度由60cm升高到75cm，已知容器底面边长为40cm，铁棒的底面边长为20cm，求铁棒的长度．
<gadget id="calculator">75 - 60</gadget>
<output>15</output>

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

<gadget id="calculator">15 / 400</gadget>
<output>3/80 = around 0.0375</output>

<gadget id="calculator">40 * 40 * (3/80)</gadget>
<output>60</output>

<result>60</result>