The students went to the zoo to play, and each of them visited at least one museum. 10 people visited the elephant house, 15 people visited the monkey house, and 6 people visited both houses. How many people went to the zoo?
同学们去动物园游玩，每人至少参观一个馆．参观大象馆的有10人，参观猴子馆的有15人，两个馆都参加的有6人，一共有多少人去动物园．
<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<gadget id="calculator">10 + 9</gadget>
<output>19</output>

<result>19</result>