There are 20 questions in one test paper, each person has 20 initial points, 4 points are awarded for each correct answer, and 1 point is deducted for each wrong answer. Archie answered all the questions, but still got 20 points. Ask: How many questions did he answer correctly?
一张试卷共有20道题目，每人都有20分的初始分，每答对一题得4分，答错一题倒扣1分，阿奇答了全部的题目，却还是20分．请问：他一共答对了几道？
<gadget id="calculator">20 - 20</gadget>
<output>0</output>

<gadget id="calculator">20 + 0</gadget>
<output>20</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

<result>4</result>