It takes 40 days to build a road that is 3 kilometers a day. If you build an extra 1 kilometer a day, how many days can it be completed in advance? (Solve with ratio)
修一条公路，每天修3千米，要40天才能完成，如果每天多修1千米，可以提前多少天完成？(用比例解)
<gadget id="calculator">40 * 3</gadget>
<output>120</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">120 / 4</gadget>
<output>30</output>

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<result>10</result>