It takes 15 hours for Zhang Hua to make a batch of parts, and 20 hours for Li Ming to do it alone. Now that the two are working together, Zhang Hua left for a period of time due to business, and it took 10 hours to complete the project. How long did Zhang Hua leave halfway?
做一批零件，张华独做需要15小时，李明独做需要20小时．现在两人合做，中途张华因事离开了一段时间，结果共用10小时完成，张华中途离开了多长时间？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) * 10</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 10</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) - 1</gadget>
<output>-1/2 = around -0.5</output>

<gadget id="calculator">(2/3) + (-1/2)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/20) - (1/20)</gadget>
<output>0</output>

<gadget id="calculator">(1/15) + 0</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/6) / (1/15)</gadget>
<output>5/2 = around 2.5</output>

<result>5/2 = around 2.5</result>