There are 6 different books, 3 with red covers and 3 with blue covers. Put the books on the shelf, and separate the books with the same cover color. So, how many different ways are there?
有6本不同的书，3本是红封面、3本是蓝封面．将这些书放在书架上，并且封面颜色相同的书要隔开．那么，不同的放法共有多少种．
<gadget id="calculator">6 * 12</gadget>
<output>72</output>

<result>72</result>