There is a project that A can complete (1/6) in 20 days by himself. B alone can complete the project in 15 days (1/4), and A and B cooperate, how many days will it take to complete the project (1/2).
有一项工程，甲单独做20天可以完成这项工程的(1/6)．乙单独做15天可以完成这项工程的(1/4)，甲、乙两人合作，需要多少天可以完成这项工程的(1/2)．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) / 20</gadget>
<output>1/120 = around 0.008333</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) / 15</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/120) + (1/60)</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">(1/2) / (1/40)</gadget>
<output>20</output>

<result>20</result>