The milk is 20 yuan per box. Due to the promotion in the mall, buy 3 boxes of milk and get 1 box free. If you buy 3 boxes at once, how much is each box cheaper?
牛奶每箱20元，因商场促销，买3箱牛奶送1箱，如果一次性买3箱，每箱便宜多少钱？
<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">20 - 15</gadget>
<output>5</output>

<result>5</result>