Students in a certain class are divided into several groups to value trees. If each group plants n trees, and n is a prime number, there are 20 saplings left; if each group plants 9 trees, there are still 2 saplings missing. How many groups are the students in this class divided into?
某班同学分成若干小组去值树，若每组植树n棵，且n为质数，则剩下树苗20棵；若每组植树9棵，则还缺少2棵树苗．这个班的同学共分成了多少组．
<gadget id="calculator">2 * 11</gadget>
<output>22</output>

<gadget id="calculator">9 - 7</gadget>
<output>2</output>

<gadget id="calculator">22 / 2</gadget>
<output>11</output>

<result>11</result>