cold drink shop stipulates that every three empty bottles can be exchanged for a bottle of original soda after drinking the soda purchased in the store. Xiao Zhang and his party of fifteen bought 21 bottles of soda, and then changed to another drink. There was no waste, so how many bottles of soda did they drink in total?
一家冷饮店规定，在本店购汽水喝完后，每三个空瓶可以换原装汽水一瓶．小张一行十五人买了21瓶汽水，之后又换喝，没有一点浪费，这样他们一共喝了多少瓶汽水．
<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">6 / 3</gadget>
<output>2</output>

<gadget id="calculator">3 / 3</gadget>
<output>1</output>

<gadget id="calculator">21 + 7 + 2 + 1</gadget>
<output>31</output>

<result>31</result>