Students in a certain class are divided into several groups to plant 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 trees missing. How many groups are the students in this class divided into?
某班同学分成若干组去植树，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，这个班的同学共分成几组？
<gadget id="calculator">20 + 2</gadget>
<output>22</output>

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

<result>11</result>