The farm harvests wheat, harvesting (2/15) of this field on the first day, (3/20) of this field on the second day, and harvesting on the third day is equal to the sum of the previous two days. How much of the land is left unclaimed?
农场收割小麦，第一天收了这块地的(2/15)，第二天收了这块地的(3/20)，第三天收的等于前两天的总和．还剩这块地的几分之几没有收？
<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>