Shampoo is 15 yuan per bottle, and the shopping malls carry out promotional activities, buy 2 bottles and get 1 bottle free. How much is it cheaper to buy 2 bottles at a time?
洗发水每瓶15元，商场开展促销活动，买2瓶送1瓶．一次买2瓶，每瓶便宜多少元？
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">15 * (2/3)</gadget>
<output>10</output>

<gadget id="calculator">15 - 10</gadget>
<output>5</output>

<result>5</result>