The class party drank a total of 21 bottles of mineral water. There are two brands of mineral water A and B in this batch of mineral water. According to statistics, on average, every 3 people drank a bottle of A mineral water, and every 4 people drank a bottle of B mineral water. How many people participated in the class party?
班级联欢会一共喝掉了21瓶矿泉水．这批矿泉水分A、B两种品牌，经统计平均每3人喝了一瓶A矿泉水，每4人喝了一瓶B矿泉水．参加班级联欢会的有多少人．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/3) + (1/4)</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">21 / (7/12)</gadget>
<output>36</output>

<result>36</result>