A drink is 2 yuan per bottle. If you buy 4 bottles and get 1 free, how much is the discount?
一种饮料每瓶2元，如果买4瓶赠送1瓶，那么就优惠多少
<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>