There are 48 students in a certain class, 20 students participate in the music interest group, 27 students participate in the art interest group, and 18 students participate in both groups. How many students participate in the interest group?
某班有学生48人，参加音乐兴趣小组的有20人，参加美术兴趣小组的有27人，两种小组都参加的有18人，参加兴趣小组的学生有多少人？
<gadget id="calculator">27 - 18</gadget>
<output>9</output>

<gadget id="calculator">20 + 9</gadget>
<output>29</output>

<result>29</result>