A project A alone can be completed in 10 days, but B alone can only be completed every day (1/15). After A started working for two days, B also joined in the project. How many days will it take to complete this project?
一项工程甲单独做10天可以完成，乙单独做每天只能完成(1/15)．甲先做两天后，乙也加入做这项工程．问完成这项工程共需几天？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * 2</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</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">(4/5) / (1/6)</gadget>
<output>24/5 = around 4.8</output>

<gadget id="calculator">(24/5) + 2</gadget>
<output>34/5 = around 6.8</output>

<result>34/5 = around 6.8</result>