Xiao Ming answered the questions by mouth. He got 6 points for a correct answer, and 3 points for a wrong answer. He answered 8 questions in total, and the result was only 21 points. How many questions did he answer correctly?
小明做口答题，答对一题得6分，答错一题扣3分，共答了8道题，结果只得21分，他答对了多少题．
<gadget id="calculator">8 * 6</gadget>
<output>48</output>

<gadget id="calculator">48 - 21</gadget>
<output>27</output>

<gadget id="calculator">3 + 6</gadget>
<output>9</output>

<gadget id="calculator">27 / 9</gadget>
<output>3</output>

<gadget id="calculator">8 - 3</gadget>
<output>5</output>

<result>5</result>