There are 13 practice questions in a test paper, 8 points are awarded for each correct answer, and 5 points are deducted for each wrong answer or non-answer. There is a student who has done all 13 questions, but the total score is 0. How many questions did this student answer correctly? ．
某试卷共有13道练习题，答对一道题得8分，答错或不答一道题扣5分，有一位同学虽然13题全部做了，但是总分为0，这位同学答对了多少道题．
<gadget id="calculator">13 * 8</gadget>
<output>104</output>

<gadget id="calculator">8 + 5</gadget>
<output>13</output>

<gadget id="calculator">104 / 13</gadget>
<output>8</output>

<gadget id="calculator">13 - 8</gadget>
<output>5</output>

<result>5</result>