road has already been repaired (2/5), and if another 400 meters are repaired, half of the road will be repaired. How many meters is this road?
一条公路已经修了它的(2/5)，再修400米，就修好这条公路的一半．这条公路长多少米？
<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>