Put 3 letters into 4 mailboxes, and each mailbox can throw at most one letter. How many different ways are there?
将3封信投到4个邮筒中，一个邮筒最多投一封信，有多少种不同的投法．
<gadget id="calculator">4 * 3 * 2</gadget>
<output>24</output>

<result>24</result>