What is the difference between the largest number and the smallest number among the four-digit numbers composed of four cards of 0, 2, 7, and 9.
用0，2，7，9四张卡片组成的四位数中，最大的数与最小的数的差是多少．
<gadget id="calculator">9_720 - 2_079</gadget>
<output>7_641</output>

<result>7_641</result>