For drinking water sanitation, the school plans to distribute a special water cup to each student. Each water cup is priced at 2 yuan, store A offers a 15% discount, store B buys nine and gets one free, and store C offers a 20% discount, which store do you think is the cheapest? How much is each cup cheaper?
为了饮水卫生，学校准备给每个学生发一个专用水杯．每个水杯标价2元，甲店优惠15%，乙店买九送一，丙店打八折，你认为到哪家商店买便宜？每个水杯便宜多少元？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">2 * (1/5)</gadget>
<output>2/5 = around 0.4</output>

<result>2/5 = around 0.4</result>