There are 40 students in class 3 of the fifth grade participating in a public opinion survey, 30 people agree with the first item, 21 people agree with the second item, 15 people agree with both items, and how many people agree with neither item ?
五年级3班有40名同学参加一项民意调查，同意第一项的有30人，同意第二项的有21人，两项都同意的有15人，两项都不同意的有多少人？
<gadget id="calculator">30 - 15</gadget>
<output>15</output>

<gadget id="calculator">21 - 15</gadget>
<output>6</output>

<gadget id="calculator">40 - 15 - 6 - 15</gadget>
<output>4</output>

<result>4</result>