Do a survey on 50 students in a class, 25 of them have been to Beijing, 20 have been to Guilin, and 10 students have been to both places, so how many students have not been to both places?
对一个班的50名同学做调查，其中有25名去过北京，20名去过桂林，有10名学生两地都去过，那么有多少名学生两地都没去．
<gadget id="calculator">20 - 10</gadget>
<output>10</output>

<gadget id="calculator">25 + 10</gadget>
<output>35</output>

<gadget id="calculator">50 - 35</gadget>
<output>15</output>

<result>15</result>