How many different three-digit numbers can be displayed with four number cards of 4, 9, 0, and 1?
用4、9、0、1四张数字卡片，能摆出多少个不同的三位数．
<gadget id="calculator">4 * 3 * 2</gadget>
<output>24</output>

<gadget id="calculator">1 * 3 * 2</gadget>
<output>6</output>

<gadget id="calculator">24 - 6</gadget>
<output>18</output>

<result>18</result>