project can be completed in 10 days by A alone, and completed by B in 15 days. Now B will do it alone for a certain number of days, and then A will join in and finish it in 4 days. How many days does B work alone?
一项工程，甲独做10天完成，乙独做15天完成，现在先由乙独做若干天后，甲再参加一起做，4天就做完了，乙独做了多少天？
<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>