Wanjiahui Supermarket has promotional activities during the New Year's Day, buy 1 box of pure milk and get 1 bottle free. There are 39 students in Class 2 of the fifth year, and each person needs to buy 3 boxes for 1 bottle. How many bottles of pure milk are there in each carton? (answer in two ways)
万家惠超市在元旦期间搞促销活动，买1箱纯牛奶另送1瓶．五年2班共有学生39人，每人分1瓶正好要买3箱．每箱纯牛奶有多少瓶？(用两种方法解答)
<gadget id="calculator">39 - 3</gadget>
<output>36</output>

<gadget id="calculator">36 / 3</gadget>
<output>12</output>

<result>12</result>