Among the 100 foreign language teachers in a district, 75 know English and 45 know Japanese. Some of them know both English and Japanese. So how many teachers only know English?
某区100名外语教师中，懂英语的75人，懂日语的45人，其中有的教师既懂英语又懂日语，那么只懂英语的教师有多少人．
<gadget id="calculator">45 - 100</gadget>
<output>-55</output>

<gadget id="calculator">75 + (-55)</gadget>
<output>20</output>

<gadget id="calculator">75 - 20</gadget>
<output>55</output>

<result>55</result>