There are 40 people participating in a knowledge contest, of which 25 people pass Chinese, 20 people pass mathematics, 10 people fail both subjects, how many people pass both subjects.
在一次知识竞赛中有40人参加，其中语文有25人及格，数学有20人及格，两科都不及格的有10人，两科都及格的有多少人．
<gadget id="calculator">40 - 10</gadget>
<output>30</output>

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

<gadget id="calculator">25 + (-10)</gadget>
<output>15</output>

<result>15</result>