In a mathematics competition among 36 students in an elementary school, 25 students answered the first question correctly, 20 students answered the second question correctly, and 15 students answered both questions correctly. How many students got both questions correct?
某小学36人在一次数学竞赛中，答对第一题的有25人，答对第二题的有20人，两题都答对的有15人，有多少个同学两题都不对？
<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">25 + 5</gadget>
<output>30</output>

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

<result>6</result>