There is a batch of goods in the warehouse, which are shipped out (1/2) on the first day, and 400 tons are shipped in on the second day. At this time, the goods in the warehouse are exactly 75% of the original. How many tons of goods were shipped on the first day?
仓库内有一批货物，第一天运出(1/2)，第二天运进400吨，这时仓库内的货物正好是原来的75%．第一天运出货物多少吨？
<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/2)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(3/4) - (1/2)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">400 / (1/4)</gadget>
<output>1_600</output>

<gadget id="calculator">1_600 * (1/2)</gadget>
<output>800</output>

<result>800</result>