There are 42 students in a fourth-grade class in a certain school, and the whole class subscribes to magazines, 38 people subscribe to "Primary School Students' World", and 24 people subscribe to "China Boy". How many people subscribe to both magazines?
某校四年级一个班有42人，全班都订了杂志，订《小学生世界》的有38人，订《中华好少年》的有24人．两种杂志都订的有多少人．
<gadget id="calculator">24 - 42</gadget>
<output>-18</output>

<gadget id="calculator">38 + (-18)</gadget>
<output>20</output>

<result>20</result>