A project needs 10 days to be done by A alone, and 15 days to be done by B alone. Now after A has finished 6 days alone, due to other urgent matters, the rest of the project is handed over to B. How many days will this project take from start to finish?
一项工程，甲独做需10天，乙独做需15天，现在甲独做6天后，因另有急事，剩下工程交给乙做，这项工程从开始到结束工需几天？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * 6</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(2/5) / (1/15)</gadget>
<output>6</output>

<gadget id="calculator">6 + 6</gadget>
<output>12</output>

<result>12</result>