The school held an on-site knowledge contest on ancient poetry. According to the rules of the competition, 20 points will be awarded for a correct answer, and 10 points will be deducted for a wrong answer. The contestants in class 61 answered 20 questions in total and scored 250 points. How many questions did the contestants in Class 1 get wrong?
学校举办古诗词现场知识竞赛．比赛规定，抢答对一题加20分，答错一题扣10分．六一班的选手共抢答了20道题，得了250分．六一班的选手答错了几道题？
<gadget id="calculator">20 * 20</gadget>
<output>400</output>

<gadget id="calculator">400 - 250</gadget>
<output>150</output>

<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">150 / 30</gadget>
<output>5</output>

<result>5</result>