A job can be completed by A alone in 15 hours, B by himself in 20 hours, and after A and B cooperate for 4 hours, the remaining work will be done by A alone, and it will take several hours to complete.
一件工作甲独做15小时完成，乙独做20小时完成，甲乙合作4小时后，剩下的工作由甲独做，还要几小时完成．
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

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

<gadget id="calculator">(1/15) + (1/20)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">(7/60) * 4</gadget>
<output>7/15 = around 0.466667</output>

<gadget id="calculator">1 - (7/15)</gadget>
<output>8/15 = around 0.533333</output>

<gadget id="calculator">(8/15) / (1/15)</gadget>
<output>8</output>

<result>8</result>