Hualian Supermarket launched a New Year's Day promotion. The original price of milk was 64 yuan per box. Now, "buy three get one free". Aunt Wang paid for three boxes at once and got four boxes of milk. How much is it cheaper than usual per box?
华联超市开展元旦促销活动，牛奶原价每箱64元，现在"买三送一"，王阿姨一次性付了3箱的钱得到了4箱牛奶，跟往常比每箱便宜多少钱？
<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">64 * (3/4)</gadget>
<output>48</output>

<gadget id="calculator">64 - 48</gadget>
<output>16</output>

<result>16</result>