For a canal, 30% of its total length was dug on the first day. If another 20 meters were dug, it would be exactly halfway dug. How long is this canal?
一条水渠，第一天挖出全长的30%，如果再挖20米，就正好挖了一半，这条水渠有多长？
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(1/2) - (3/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">20 / (1/5)</gadget>
<output>100</output>

<result>100</result>