16 yuan per bottle of drink, buy 3 get 1 free. Buying 3 bottles at a time, how much is the actual cost of a bottle?
每瓶饮料16元，买3送1．一次买3瓶，实际一瓶便宜多少元？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">16 / 4</gadget>
<output>4</output>

<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">16 - 12</gadget>
<output>4</output>

<result>4</result>