40 people participated in the test, 30 people answered the first question correctly, 21 people answered the second question correctly, 15 people answered both questions correctly, and how many people answered neither question correctly?
40人参加测验，答对第一题的有30人，答对第二题的有21人，两题都答对的有15人，两题都没答对的有多少人？
<gadget id="calculator">21 - 15</gadget>
<output>6</output>

<gadget id="calculator">30 + 6</gadget>
<output>36</output>

<gadget id="calculator">40 - 36</gadget>
<output>4</output>

<result>4</result>