There are 56 students in Class 1 of the fifth grade, each of whom has completed at least one kind of homework, 45 of them have finished their math homework, and 42 of them have finished their Chinese homework. How many people in this class have finished both homework.
五年级1班有学生56人，其中每人至少做完一种作业，45人做完数学作业，42人做完语文作业，这个班两种作业都做完的人有多少人．
<gadget id="calculator">42 - 56</gadget>
<output>-14</output>

<gadget id="calculator">45 + (-14)</gadget>
<output>31</output>

<result>31</result>