When 40 students are doing three math problems, 25 people get the first problem right, 28 people get the second problem right, and 31 people get the third problem right, so at least how many people get the three problems right ．
40名同学在做3道数学题时，有25人做对第一题，有28人做对第二题，有31人做对第三题，那么至少有多少人做对了三道题的．
<gadget id="calculator">40 - 25</gadget>
<output>15</output>

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

<gadget id="calculator">40 - 31</gadget>
<output>9</output>

<gadget id="calculator">40 - 15 - 12 - 9</gadget>
<output>4</output>

<result>4</result>