For a project, if A alone takes 30 days, B alone takes 20 days, and A and B cooperate for 4 days. How much of the project is left?
一项工程，若甲独做需要30天，乙独做需要20天，在甲、乙合作了4天．还剩下工程的几分之几？
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/30) + (1/20)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 4</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

<result>2/3 = around 0.666667</result>