It takes 10 days for team A to complete a project alone, and 15 days for team B to complete it alone. A will do the work for 4 days first, and then team B will continue to do the rest. How many days will it take to complete?
一项工程，甲队单独完成要10天，乙队单独完成要15天，甲先做4天后，剩下的由乙队接着做，还要几天才能完成？
<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>