The Experimental Primary School held a mathematics competition with a total of 20 questions. If you answer a question correctly, you get 5 points, and if you answer a question incorrectly or do not answer a question, 2 points are deducted. Liu Yang scored 79 points, how many questions did he get wrong?
实验小学举行数学竞赛，共20道题．若做对一题得5分，做错或没做一题扣2分．刘洋得了79分，他做错了几道题？
<gadget id="calculator">5 * 20</gadget>
<output>100</output>

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

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

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

<result>3</result>