A building has 21 floors, 5 units in total, and 3 households on each floor. How many households are there in this building?
一栋楼房有21层，共5个单元，每层住3户，这座楼一共有多少户？
<gadget id="calculator">21 * 3 * 5</gadget>
<output>315</output>

<result>315</result>