There are 40 people in a travel agency, 24 of them can speak English, and 20 can speak Russian. There are 5 people who know neither, how many people know both?
一个旅行社有40人，其中会英语的有24人，会俄语的有20人．两样都不会的有5人，两样都会的有多少人？
<gadget id="calculator">40 - 5</gadget>
<output>35</output>

<gadget id="calculator">20 - 35</gadget>
<output>-15</output>

<gadget id="calculator">24 + (-15)</gadget>
<output>9</output>

<result>9</result>