There is a project that A alone takes 15 days and B alone takes 20 days. If A works for a certain number of days first, and B continues to do it in 18 days, how many days does A do it?
有一件工程，甲单独做需15天，乙单独做需20天．若甲先做若干天后，乙接着做，共用18天完成，甲做了几天？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">18 * (1/15)</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">(6/5) - 1</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">(1/15) - (1/20)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/5) / (1/60)</gadget>
<output>12</output>

<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<result>6</result>