Supermarkets carry out preferential activities, laundry detergent is 25 yuan per bottle, buy 4 bottles and get 1 bottle free. Mom bought 4 bottles at a time, how much is each bottle cheaper on average?
超市开展优惠活动，洗衣液每瓶25元，买4瓶送1瓶．妈妈一次买了4瓶，平均每瓶便宜了多少钱？
<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>