A book of extra-curricular math exercises can be completed by A in 6 hours and B in 12 hours; A in 8 hours and B in 6 hours. If A does 3 hours and B continues to do it, how many hours will it take to complete?
一本数学课外练习，甲做6小时，乙做12小时可完成；甲做8小时，乙做6小时也可以完成，如果甲做3小时后由乙接着做，还需多少小时才能完成．
<gadget id="calculator">12 - 6</gadget>
<output>6</output>

<gadget id="calculator">8 - 6</gadget>
<output>2</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<gadget id="calculator">18 + 12</gadget>
<output>30</output>

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

<gadget id="calculator">(1/30) * 3 * 3</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">(7/10) / (1/30)</gadget>
<output>21</output>

<result>21</result>