A bamboo pole is 500 cm long. Put it vertically into the river and make a mark along the water surface. Then turn the bamboo pole upside down and put it vertically into the river. Make a mark along the water surface. It is known that the distance between the two marks is 40 cm and the depth of the river centimeter?
一根竹竿长500厘米，把它垂直放入河中，沿水面做个记号，接着把竹竿倒过来再垂直放入河中，也沿水面做个记号，已知两个记号相距40厘米，河深多少厘米？
<gadget id="calculator">500 - 40</gadget>
<output>460</output>

<gadget id="calculator">460 / 2</gadget>
<output>230</output>

<result>230</result>