For a highway, the ratio of the completed section to the total length is 1:3, and after 400 kilometers are repaired, the ratio of the completed section to the remaining length is 3:2. How long is this highway in kilometers?
一条公路，已修的与全长的比是1：3，再修400千米，已修的与剩下的比是3：2，这条公路长多少千米？
<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(3/5) - (1/3)</gadget>
<output>4/15 = around 0.266667</output>

<gadget id="calculator">400 / (4/15)</gadget>
<output>1_500</output>

<result>1_500</result>