To build a road, the full length (2/15) is repaired on the first day, the full length (3/20) is repaired on the second day, and the repaired road on the third day is equal to the sum of the previous two days. How much is left unrepaired?
修一条公路，第一天修了全长的(2/15)，第二天修了全长的(3/20)，第三天修的等于前两天的总和，还剩多少没有修？
<gadget id="calculator">2 / 15</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">3 / 20</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(2/15) + (3/20)</gadget>
<output>17/60 = around 0.283333</output>

<gadget id="calculator">(2/15) + (3/20) + (17/60)</gadget>
<output>17/30 = around 0.566667</output>

<gadget id="calculator">1 - (17/30)</gadget>
<output>13/30 = around 0.433333</output>

<result>13/30 = around 0.433333</result>