How many ways are there to put 4 different balls into 4 different boxes, one in each box?
把4个不同的球放入4个不同的盒子中，每一个盒子里有一个球，有多少种放法？
<gadget id="calculator">4 * 3 * 2 * 1</gadget>
<output>24</output>

<result>24</result>