A milk product factory sells a batch of milk. It sells (2/5) of the total on the first day and (3/5) of the total on the second day. The difference between the two days' sales is 400 boxes. How many cartons are there in this batch of milk?
某牛奶制品厂出售一批牛奶，第一天卖出总数的(2/5)，第二天卖出总数的(3/5)，两天的销售量相差400箱．这批牛奶共有多少箱？
<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

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

<gadget id="calculator">(3/5) - (2/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">400 / (1/5)</gadget>
<output>2_000</output>

<result>2_000</result>