In a certain school, 21 students participated in the mathematics interest group, 15 students participated in the science group, and three times as many students participated in the composition group as the mathematics and science groups. How many people participated in the composition interest group?
某校学生参加数学兴趣小组的有21人，参加科学小组的有15人，参加作文小组的是数学和科学小组人数的3倍，参加作文兴趣小组的有几人？
<gadget id="calculator">21 + 15</gadget>
<output>36</output>

<gadget id="calculator">36 * 3</gadget>
<output>108</output>

<result>108</result>