The original price of a kind of chocolate is 20 yuan a box, and the shopping mall is carrying out promotional activities. Xinxin mall buys 3 boxes and gets one free, and Shengli mall buys 1 box and gets a discount of 4 yuan. How much is a box cheaper? Please list the calculation.
一种巧克力原价是20元一盒，商场开展促销活动，欣欣商场买3盒送一盒，胜利商场买一盒减4元，到哪个商场买便宜？一盒便宜多少元？请列式计算．
<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">20 * (3/4)</gadget>
<output>15</output>

<gadget id="calculator">16 - 15</gadget>
<output>1</output>

<result>1</result>