To celebrate Christmas and New Year’s Day, the two supermarkets have different promotions for Wangzai’s reconstituted milk: Qiyi Supermarket buys four boxes for two yuan and gets one free; Huanggang Supermarket’s original price of 1.8 yuan is now reduced (1/4). Xiao Ming is going to buy 20 boxes of milk, which supermarket is the cheapest? How much does it cost in total?
迎圣诞元旦双节做活动，两超市对旺仔复原乳牛奶作出不同的促销活动：七一超市每盒两元买四盒送一盒；黄冈超市原价1.8元现在降价(1/4)出售．小明准备买20盒牛奶，去哪个超市便宜？一共要多少钱？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1.8 * (3/4) * 20</gadget>
<output>27</output>

<result>27</result>