A batch of notebooks were distributed to Class 51 students, with an average of 12 copies per person. If only girls were distributed, each person would get 20 copies on average. If only boys were distributed, how many copies per person would be distributed on average.
一批本子发给五一班同学，平均每人12本，若只发给女生，平均每人可分到20本，若只分给男生，平均每人分得多少本．
<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>