project is completed in 12 days by A alone, 15 days by B alone, and B will continue after 4 days by A alone. How many days will it take to complete?
一项工程，甲单独做12天完成，乙单独做15天完成，甲单独做4天后由乙接着做，还需几天完成？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 4</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

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

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

<result>10</result>