There are 50 students in a class, 45 of them like computers, 35 of them like art, and 40 of them like music. How many of them like these three at the same time?
某班有50名学生，喜欢电脑的有45人，喜欢美术的有35人，喜欢音乐的有40人，求同时喜欢这三样的有几人？
<gadget id="calculator">50 - 45</gadget>
<output>5</output>

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

<gadget id="calculator">50 - 40</gadget>
<output>10</output>

<gadget id="calculator">5 + 15 + 10</gadget>
<output>30</output>

<gadget id="calculator">50 - 30</gadget>
<output>20</output>

<result>20</result>