An elementary school participated in a school district mathematics competition. The average score of 6 students in the upper grade is 71 points, the average score of 7 students in the middle grade is 64 points, and the average score of 8 students in the lower grade is 64 points. Find the average score of the participating students.
某小学参加学区数学竞赛，高年级6人平均分数是71分，中年级7人平均分数是64分，低年级8人平均分数是64分，求参赛同学的平均分数．
<gadget id="calculator">71 * 6</gadget>
<output>426</output>

<gadget id="calculator">64 * 7</gadget>
<output>448</output>

<gadget id="calculator">64 * 8</gadget>
<output>512</output>

<gadget id="calculator">426 + 448 + 512</gadget>
<output>1_386</output>

<gadget id="calculator">6 + 7 + 8</gadget>
<output>21</output>

<gadget id="calculator">1_386 / 21</gadget>
<output>66</output>

<result>66</result>