The building where Xiaohong lives has 21 floors and 5 units, and each unit lives in 3 households on each floor. How many households are there in this building?
小红住的这座楼有21层5个单元，每个单元每层住3户．这座楼里一共有多少户？
<gadget id="calculator">3 * 21 * 5</gadget>
<output>315</output>

<result>315</result>