How many different four-digit numbers can be formed by using four number cards with 1, 2, 3, and 4 written on them.
用写有1、2、3、4的四张数字卡片可以摆出多少个不同的四位数．
<gadget id="calculator">4 * 3 * 2 * 1</gadget>
<output>24</output>

<result>24</result>