Three people participate in social practice activities. A and B each bring 18 yuan on average, and A, B, and C each bring 21 yuan on average. How much yuan does C bring?
三个人参加社会实践活动，甲、乙平均每人带18元，甲、乙、丙三个人平均每人带21元，丙带了多少元？
<gadget id="calculator">21 * 3</gadget>
<output>63</output>

<gadget id="calculator">18 * 2</gadget>
<output>36</output>

<gadget id="calculator">63 - 36</gadget>
<output>27</output>

<result>27</result>