A job is planned to be completed in a certain number of days. A alone will complete it 4 days earlier than planned, and B alone will complete it 6 days later than planned. This work is done by A and B alone after 2 days, and it happens to be completed on time. How many days will it take to complete this work if A and B cooperate with each other from the beginning?
一项工作计划一定天数完成，由甲独做则比计划早4天完成，由乙独做则比计划晚6天完成．这项工作由甲乙合作2天后由乙独做恰好按时完成，这项工作若一开始就由甲乙二人合作需要多少天完成？
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

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

<gadget id="calculator">(1/5) + (1/15)</gadget>
<output>4/15 = around 0.266667</output>

<gadget id="calculator">1 / (4/15)</gadget>
<output>15/4 = around 3.75</output>

<result>15/4 = around 3.75</result>