Randomly take three cards from the four cards 0, 1, 2, and 3 to form a number. How many different ways are there in total?
从0、1、2、3四张卡片中任意取出三张，组成一个数，一共有多少种不同取法．
<gadget id="calculator">3 * 6</gadget>
<output>18</output>

<result>18</result>