A project can be completed by team A alone in 10 days and by team B in 15 days alone. If Team A works for 4 days first, and Team B does the rest, how many days will it take to complete the project?
一项工程，甲队单独做10天可以完成，乙队单独做15天可以完成．如果甲队先做4天，剩下的由乙队去做，这项工程还要几天可以完成？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * 4</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

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

<gadget id="calculator">(3/5) / (1/15)</gadget>
<output>9</output>

<result>9</result>