There are 40 students in a class, 25 of them have passed Chinese, 28 have passed English, and 30 have passed mathematics. How many students have passed all three subjects at least?
某班别共有学生40人，其中25人中文及格，28人英文及格，30人数学及格，问最少有多少个学生三科均及格﹖
<gadget id="calculator">40 - 25</gadget>
<output>15</output>

<gadget id="calculator">40 - 28</gadget>
<output>12</output>

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<gadget id="calculator">40 - 15 - 12 - 10</gadget>
<output>3</output>

<result>3</result>