When 50 students are doing three math problems, 36 people get the first problem right, 29 people get the second problem right, and 42 people get the third problem right, so at least how many people get the three problems right at the same time ．
50名同学在做3道数学题时，有36人做对第一题，有29人做对第二题，有42人做对第三题，那么至少有多少人同时做对了三道题．
<gadget id="calculator">50 - 36</gadget>
<output>14</output>

<gadget id="calculator">50 - 29</gadget>
<output>21</output>

<gadget id="calculator">50 - 42</gadget>
<output>8</output>

<gadget id="calculator">50 - 14 - 21 - 8</gadget>
<output>7</output>

<result>7</result>