Shampoo is 15 yuan per bottle, and the shopping malls carry out promotional activities, buy 4 bottles and get 1 bottle free. How much is it cheaper to buy 4 bottles at a time?
洗发水每瓶15元，商场开展促销活动，买4瓶送1瓶．一次买4瓶，每瓶便宜多少元？
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">15 * (4/5)</gadget>
<output>12</output>

<gadget id="calculator">15 - 12</gadget>
<output>3</output>

<result>3</result>