For drinking water sanitation, the school plans to distribute a special water cup to each student. Huijia Supermarket offers "15% discount" and Convenience Supermarket "buy four get one free". Each water cup is priced at 2 yuan, which merchant do you think is cheaper to buy? How much is each drinking glass cheaper?
为了饮水卫生，学校准备给每个学生发一个专用水杯．惠佳超市"八五折优惠"，便民超市"买四送一"．每个水杯表标价2元，你认为到哪家商家买便宜？每个水杯便宜多少？
<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 - (3/20)</gadget>
<output>17/20 = around 0.85</output>

<gadget id="calculator">2 * (17/20)</gadget>
<output>17/10 = around 1.7</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">4 * (2/5)</gadget>
<output>8/5 = around 1.6</output>

<gadget id="calculator">(17/10) - (8/5)</gadget>
<output>1/10 = around 0.1</output>

<result>1/10 = around 0.1</result>