To build a road, the ratio of the length of the repaired road to the length of the road that has not been repaired is 1:5. After another 490 meters have been repaired, the ratio of the length of the road that has been repaired to the length of the road that has not been repaired is 3:1. What is the length of the road that has not been repaired?
修一条公路，已修长度和未修长度的比是1：5，又修了490米后，已修长度和未修长度的比是3：1，这时未修公路的长度为多少米．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 + 5</gadget>
<output>6</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(3/4) - (1/6)</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">490 / (7/12)</gadget>
<output>840</output>

<gadget id="calculator">840 / 4</gadget>
<output>210</output>

<gadget id="calculator">210 * 1</gadget>
<output>210</output>

<result>210</result>