There are 50 people in class 3 and 1 participating in the Chinese interest group and mathematics interest group, of which 31 people participated in the Chinese interest group, 35 people participated in the mathematics interest group, how many people participated in both interest groups.
三1班有50人参加了语文兴趣小组和数学兴趣小组，其中有31人参加了语文兴趣小组，35人参加了数学兴趣小组，两个兴趣小组都参加的有多少人．
<gadget id="calculator">31 + 35</gadget>
<output>66</output>

<gadget id="calculator">66 - 50</gadget>
<output>16</output>

<result>16</result>