A job is completed in 10 days by A alone, and 15 days by B alone. Now A and B cooperate for 4 days, and B will do it alone. How many days will it take for B to complete it?
一项工作，甲单独做10天完成，乙独做15天完成，现在甲、乙合作4天后，由乙独做，乙还需几天才能完成？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) * 4</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 - (2/3)</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(1/3) / (1/15)</gadget>
<output>5</output>

<result>5</result>