A batch of parts A alone takes 6 days to complete, and B alone takes 4 days to complete. If A works for 2 days first and then B joins in, how many days will it take for the two to finish?
一批零件甲单独做6天完成，乙独做要4天完成．如果甲先做2天后，乙加入一起做，两人要几天才能完成？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) * 2</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/6) + (1/4)</gadget>
<output>5/12 = around 0.416667</output>

<gadget id="calculator">(2/3) / (5/12)</gadget>
<output>8/5 = around 1.6</output>

<result>8/5 = around 1.6</result>