75% of the total length of a canal has been built, and it is still 3 kilometers away from the midpoint. How long is this canal in kilometers?
修一条水渠，已经修了全长的75%，现在离中点还有3千米，这条水渠长多少千米？
<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">(3/4) - (1/2)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">3 / (1/4)</gadget>
<output>12</output>

<result>12</result>