A certain school held a mathematics competition. There were 20 questions in total, 6 points were awarded for each correct answer, and 3 points were deducted for each wrong answer. Xiao Cong scored 57 points in total. How many questions did he miss?
某校进行数学竞赛，共有题20道，做对一道得6分，做错一道倒扣3分．小聪共得57分，他错了多少题．
<gadget id="calculator">20 * 6</gadget>
<output>120</output>

<gadget id="calculator">120 - 57</gadget>
<output>63</output>

<gadget id="calculator">6 + 3</gadget>
<output>9</output>

<gadget id="calculator">63 / 9</gadget>
<output>7</output>

<result>7</result>