There are 9 number cards, which are respectively written 1, 2, 3, ... 9. Each card can form a two-digit number. How many different two-digit numbers can be formed by these 9 cards.
有数字卡片9张，分别写着1，2，3，…9，每张卡片可以组成一个两位数，这9张卡片可以组成多少个不同的两位数．
<gadget id="calculator">9 * 8</gadget>
<output>72</output>

<result>72</result>