To build a road, the first time to repair the whole (1/4), and the second time to repair 20% of the whole way, at this time there is still 30 kilometers away from the middle point of the road. How many kilometers is this road?
修一条公路，第一次修全程的(1/4)，第二次又修了全程的20%，这时离公路中点还有30千米．这条公路长多少千米？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) + (1/5)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">(1/2) - (9/20)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">30 / (1/20)</gadget>
<output>600</output>

<result>600</result>