There are 21 boys in the art group of the school, and the number of music interest groups is four times that of the art group. How many music interest groups are there?
学校美术小组有男生21人，音乐兴趣小组的人数是美术小组的4倍，音乐兴趣小组有多少？
<gadget id="calculator">21 * 4</gadget>
<output>84</output>

<result>84</result>