To dig a canal, the full length (3/5) was dug on the first day, and the full length (1/5) was dug on the second day, leaving 4 kilometers undug. How long is this canal in kilometers?
挖一条水渠，第一天挖了全长的(3/5)，第二天挖了全长的(1/5)，还剩4千米的没挖．这条水渠全长多少千米？
<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (3/5) - (1/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">4 / (1/5)</gadget>
<output>20</output>

<result>20</result>