book has a total of 240,000 words. If A alone takes 20 hours to type, and if A and B take 12 hours to type, how many hours does B need to type alone?
一本书共24万字，如果甲单独打要20h，如果甲、乙合打要12h，那么乙单独打要多少h？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

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

<gadget id="calculator">(1/12) - (1/20)</gadget>
<output>1/30 = around 0.033333</output>

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

<result>30</result>