A train from Shanghai to Hankou has already passed (2/5), and there are still 400 kilometers away from the midpoint. How many kilometers is the railway from Shanghai to Hankou?
一列火车从上海开往汉口，已经行了(2/5)，离中点还有400千米．上海到汉口的铁路长多少千米？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/2) - (2/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">400 / (1/10)</gadget>
<output>4_000</output>

<result>4_000</result>