For a road, the ratio of the completed length to the total length is 1:3, and after another 20 kilometers are built, the ratio of the completed length to the total length is 2:3. How many kilometers is this road? (solution by equation)
一条公路，已修的与全长的比是1：3，再修20千米，已修的与全长的比是2：3，这条公路长多少千米？(用方程解)
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(2/3) - (1/3)</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">20 / (1/3)</gadget>
<output>60</output>

<result>60</result>