The entire engineering team built a road. Last month, half of the road was repaired. This month, due to the rain, only 21% of the road was repaired. At this time, the repaired road is 8.4 kilometers longer than the rest. How long is this road in kilometers?
全工程队修一条公路，上个月已修了公路的一半，这个月因下雨只修了公路的21%，这时已修好的比剩下的多8.4千米，这条公路长多少千米？
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">21 / 100</gadget>
<output>21/100 = around 0.21</output>

<gadget id="calculator">(1/2) + (21/100)</gadget>
<output>71/100 = around 0.71</output>

<gadget id="calculator">1 - (71/100)</gadget>
<output>29/100 = around 0.29</output>

<gadget id="calculator">(21/100) - (29/100)</gadget>
<output>-2/25 = around -0.08</output>

<gadget id="calculator">(1/2) + (-2/25)</gadget>
<output>21/50 = around 0.42</output>

<gadget id="calculator">8.4 / (21/50)</gadget>
<output>20</output>

<result>20</result>