A certain brand of shampoo is 25 yuan per bottle, and the supermarket implements a promotional activity to buy 4 and get 1 free. Aunt Wang bought 4 bottles at a time, how much is each bottle cheaper on average?
某种品牌的洗发水每瓶25元，超市实行促销活动买4送1．王阿姨一次性买4瓶，平均每瓶便宜多少元？
<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">25 * (4/5)</gadget>
<output>20</output>

<gadget id="calculator">25 - 20</gadget>
<output>5</output>

<result>5</result>