A kind of mineral water sells for 2 yuan per bottle in retail. A shopping mall launched a big bargain activity of "buy four and get one free". How much is the discount?
一种矿泉水，零售每瓶卖2元，某商场开展"买四赠一"大酬宾活动，优惠了多少．
<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">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<result>1/5 = around 0.2</result>