There are 20 questions in the math competition, 5 points for each correct answer, and 2 points for each wrong answer (or no answer). Xiao Ming has a total of 79 points. How many questions did he answer wrong?
数学竞赛，共20道题，每答对一道的5分，每答错(或不答)一道扣2分，小明共得79分，他答错了几道题？
<gadget id="calculator">20 * 5</gadget>
<output>100</output>

<gadget id="calculator">100 - 79</gadget>
<output>21</output>

<gadget id="calculator">5 + 2</gadget>
<output>7</output>

<gadget id="calculator">21 / 7</gadget>
<output>3</output>

<result>3</result>