A project is completed in 20 days by the cooperation of teams A and B, 60 days by the cooperation of teams B and C, and 30 days by the cooperation of teams C and D, how many days will it be completed by the cooperation of A and D?
一项工程，甲、乙两队合作20天完成，乙丙两队合作60天完成，丙丁两队合作30完成，甲丁合作多少天完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/30) - (1/60)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/20) + (1/60)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / (1/15)</gadget>
<output>15</output>

<result>15</result>