A cold drink shop stipulates that every three empty bottles can be exchanged for a bottle of original soda after the soda purchased in the store is finished. Xiao Zhang and his party of 15 people bought 21 bottles of soda, and then exchanged the empty bottles for soda to drink. There was no waste. How many bottles of soda did they drink in total?
一家冷饮店规定在本店购买汽水喝完后，每三个空瓶可以换原装汽水一瓶．小张一行15人买了21瓶汽水，之后又用空瓶换汽水喝，没有一点浪费．这样他们一共喝了多少瓶汽水．
<gadget id="calculator">21 + 7 + 2 + 1</gadget>
<output>31</output>

<result>31</result>