The fourth grade chorus team organized singing, 12 people went to class 1, 18 people went to class 2, and 21 people went to class 3. How many people sang in each class on average?
四年级合唱队组织唱歌，1班去了12人，2班去了18人，3班去了21人，平均每班唱歌去了多少人？
<gadget id="calculator">12 + 18 + 21</gadget>
<output>51</output>

<gadget id="calculator">51 / 3</gadget>
<output>17</output>

<result>17</result>