A cross-country race is held in a certain city. When running (2/5) of the full length, there is still 3km away from the midpoint. How many kilometers is the total length of the cross-country race?
某城市举行越野比赛，跑了全长的(2/5)时，离中点还有3km，越野赛程全长多少千米？
<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">3 / (1/10)</gadget>
<output>30</output>

<result>30</result>