A project can be completed by A alone in 10 days and by B in 15 days. Now A works alone for 4 days, and B works alone for 3 days. For the rest of the project, A and B work together. How many days will it take to complete this project?
一项工程，甲独做10天完成，乙独做15天完成．现在甲先单独做4天后，乙又单独做3天，余下的工程甲、乙合做，还要多少天完成这项工程？
<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 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) * 3</gadget>
<output>1/5 = around 0.2</output>

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

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

<gadget id="calculator">(2/5) / (1/6)</gadget>
<output>12/5 = around 2.4</output>

<result>12/5 = around 2.4</result>