For the students to plant turf, it takes 20 hours for the first class and 30 hours for the second class. After 10 hours of combined work, the second class will do the rest alone. How many hours will it take?
同学们植草皮，一班需要20小时完成，二班需30小时完成，两班合干10小时后，余下的由二班独做，还需多少小时完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

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

<gadget id="calculator">(1/12) * 10</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">1 - (5/6)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) / (1/30)</gadget>
<output>5</output>

<result>5</result>