Shengli Primary School held an environmental protection knowledge contest, with a total of 20 questions. The scoring standard is: 5 points for a correct answer, 2 points for a wrong answer or no answer. Xiao Ming scored 79 points, how many questions did he answer correctly?
胜利小学举行环保知识竞赛，共20道题，评分标准是：答对一题得5分，答错或没答扣2分．小明得了79分，他答对了几题？
<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>

<gadget id="calculator">20 - 3</gadget>
<output>17</output>

<result>17</result>