In the mid-term final exam of class 41, 17 people got excellent in Chinese, 20 people got excellent in mathematics, and the students who got "excellent" by the teacher all stood up and counted 30 people. How many people got "excellent" in both subjects?
四1班同学中期末考试中，语文得优的有17人，数学得优的有20人，老师请得"优"的同学都站起来，数了数有30人．两科都得"优"的有几人？
<gadget id="calculator">20 - 30</gadget>
<output>-10</output>

<gadget id="calculator">17 + (-10)</gadget>
<output>7</output>

<result>7</result>