There are currently 50 students doing physics and chemistry experiments. If 40 students do the physics experiment correctly, 31 students do the chemistry experiment correctly, and 4 students do both experiments correctly, then both experiments are done correctly. how many.
现有50名学生都做物理、化学实验，如果物理实验做正确的有40人，化学实验做正确的有31人，两种实验都做错的有4人，则两种实验都做对的有多少．
<gadget id="calculator">50 - 4</gadget>
<output>46</output>

<gadget id="calculator">31 - 46</gadget>
<output>-15</output>

<gadget id="calculator">40 + (-15)</gadget>
<output>25</output>

<result>25</result>