For a project, it takes 12 days for teams A, B, and C to work together, and 30 days for team A to do it alone. Now teams B and C are working together, how many days will it take?
一项工程，由甲、乙、丙三队合做要12天，甲队单独做要30天．现在由乙、丙两队合做，需要多少天？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

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

<gadget id="calculator">(1/12) - (1/30)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / (1/20)</gadget>
<output>20</output>

<result>20</result>