A project can be completed by A alone in 12 hours. After A and B cooperate for 4 hours, B spends another 6 hours to complete the project. How many hours will it take for B to complete it alone?
一项工程，甲单独做12小时完成，甲乙合作4小时后，乙又用了6小时完成这项工程，则乙单独做需多少小时完成？
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">(2/3) / 10</gadget>
<output>1/15 = around 0.066667</output>

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

<result>15</result>