How many different four-digit numbers can be formed by using four number cards of 7, 2, 5, and 9.
用7、2、5、9四张数字卡片可以排出多少个不同的四位数．
<gadget id="calculator">4 * 3 * 2 * 1</gadget>
<output>24</output>

<result>24</result>