There are 25 people in the dance group, 20 people in the chorus group, 10 people in both, and 15 people in neither. How many people are there in the third grade?
参加舞蹈组的有25人，参加合唱组的有20人，两项都参加的有10人，两项都没有参加的有15人，三年级一共有多少人？
<gadget id="calculator">20 - 10</gadget>
<output>10</output>

<gadget id="calculator">25 + 10 + 15</gadget>
<output>50</output>

<result>50</result>