The school evenly divided the campus green area into two classes of the sixth grade to clean up. It took Class 1 and Class 6 15 minutes to complete it, and Class 2 and Class 6 took 20 minutes to complete. How many minutes can it be completed if two shifts work together?
学校把校园绿地平均分给六年级两个班清理，六1班用了15分钟完成，六2班用了20分钟完成．如果两班合做几分钟可以完成？
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

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

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

<gadget id="calculator">(1/15) + (1/20)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">2 / (7/60)</gadget>
<output>120/7 = around 17.142857</output>

<result>120/7 = around 17.142857</result>