A school has 120 students who are allowed to take three courses, A, B, and C. There are 60 people who choose A, 65 people who choose B, 55 people who choose C, 30 people who choose A and B, 40 people who choose B and C, and 35 people who choose C and A , and the number of people who choose A, B, and C is unknown. So how many students are there at least who have not chosen any of the three subjects?
某校有120名学生，允许选修A、B、C三门课．选修A的有60人，选修B的有65人，选修C的有55人，兼选A、B的有30人，兼选B、C的有40人，兼选C、A的有35人，而A、B、C都选的人数不详．那么三科都没有选的学生最少有多少人？
<gadget id="calculator">45 - 30</gadget>
<output>15</output>

<result>15</result>