To build a canal, the full length (3/7) has been built, and now there is still 3 kilometers away from the midpoint. How many kilometers is this water?
修一条水渠，已经修了全长的(3/7)，现在离中点还有3千米，这条水娶长多少千米？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(1/2) - (3/7)</gadget>
<output>1/14 = around 0.071429</output>

<gadget id="calculator">3 / (1/14)</gadget>
<output>42</output>

<result>42</result>