There are 50 students in a class. In a test, 25 people get the first question right, 20 people get the second question right, and 10 people get both questions right. How many students get neither of the two questions right?
某班有50名同学，在一次测验中答对第一题的有25人，答对第二题的有20人，两题都答对的有10人，则有多少名同学两题都不对．
<gadget id="calculator">50 - 25 - 20</gadget>
<output>5</output>

<gadget id="calculator">5 + 10</gadget>
<output>15</output>

<result>15</result>