To clean the classroom, it takes 30 minutes for A to clean alone, and 20 minutes for B to clean alone. After A sweeps for 3 minutes, B comes to help him sweep together, how many minutes will it take to finish sweeping?
打扫教室的卫生，甲单独扫要30分钟，乙单独扫要20分钟．甲扫了3分钟后，乙来帮他一起扫，还要几分钟扫完？
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/30) * 3</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

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

<gadget id="calculator">(1/30) + (1/20)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(9/10) / (1/12)</gadget>
<output>54/5 = around 10.8</output>

<result>54/5 = around 10.8</result>