The street needs to replace a batch of water pipes. It is known that each new water pipe is 8 meters long, and each original old water pipe is 5 meters long. Now there are 400 new water pipes in total, how many old water pipes can be replaced?
街道要更换一批水管，已知新水管每根长8米，原有的旧水管每根长5米．现在有新水管共400根，可以换掉多少根旧水管？
<gadget id="calculator">400 / 5</gadget>
<output>80</output>

<gadget id="calculator">8 * 80</gadget>
<output>640</output>

<result>640</result>