Everyone in the chess interest group of Class 3 and 2 can play at least one kind of chess. There are 21 students who can play chess, 17 students who can play Go, and 10 students who can play both kinds of chess. How many students are there in this group?
三2班棋类兴趣小组中每人至少会下一种棋，会下象棋的有21名同学，会下围棋的有17名，两种棋都会的有10名．这个组共有多少名同学？
<gadget id="calculator">17 - 10</gadget>
<output>7</output>

<gadget id="calculator">21 + 7</gadget>
<output>28</output>

<result>28</result>