A batch of books is distributed to a certain class of students, and each person gets 12 books on average. If it is only distributed to girls in this class, each person can get 30 books on average. If it is only distributed to boys in this class, how many books per person can get on average.
一批书分给某班学生，平均每人得12本，如果只发给该班女生，平均每人可得30本，如果只发给该班男生，平均每人可得多少本．
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

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

<gadget id="calculator">(1/12) - (1/30)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / (1/20)</gadget>
<output>20</output>

<result>20</result>