Xiaohong takes part in a science test. There are 20 true and false questions in total. She gets 5 points for a correct answer and 2 points for a wrong answer. Xiaohong scored 79 points. How many questions did Xiaohong get right?
小红参加一次科学测验，一共有20道是非题，对一题得5分，错一题扣2分．小红得了79分，小红做对了几道题？
<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>

<gadget id="calculator">20 - 3</gadget>
<output>17</output>

<result>17</result>