Dongfanghong Elementary School held a mathematics competition, with 20 questions in total, 5 points for each correct answer, and 5 points deducted for each wrong question. Zhang Ming did all the questions and got 70 points in total. He got a few questions wrong ?
东方红小学举行数学竞赛，共20道题，每做对一题得5分，做错一题倒扣5分，张明做了所有的题，共得70分，他做错了几道题？
<gadget id="calculator">20 * 5</gadget>
<output>100</output>

<gadget id="calculator">100 - 70</gadget>
<output>30</output>

<gadget id="calculator">5 + 5</gadget>
<output>10</output>

<gadget id="calculator">30 / 10</gadget>
<output>3</output>

<result>3</result>