Class 1 held a campus safety knowledge competition. 10 points for 1 correct answer, 5 points deducted for 1 wrong answer. The first group grabbed 15 questions in total and scored 120 points. How many questions did this group answer correctly?
六1班举行校园安全知识抢答竞赛．答对1题得10分，答错1题倒扣5分．第一小组共抢到15题，得了120分，这个小组答对了多少题．
<gadget id="calculator">10 * 15</gadget>
<output>150</output>

<gadget id="calculator">150 - 120</gadget>
<output>30</output>

<gadget id="calculator">10 + 5</gadget>
<output>15</output>

<gadget id="calculator">30 / 15</gadget>
<output>2</output>

<gadget id="calculator">15 - 2</gadget>
<output>13</output>

<result>13</result>