There are 56 students in Class 5 and 2. In a test, 34 students answered the first question correctly, 29 students answered the second question correctly, and 15 students answered both questions correctly.
五2班有56个学生，在一次测验中，答对第一题的34人，答对第二题的29人，两题都答对的15人，那么，两题都不对的有多少人．
<gadget id="calculator">56 - 34 - 29</gadget>
<output>-7</output>

<gadget id="calculator">(-7) + 15</gadget>
<output>8</output>

<result>8</result>