A batch of books, after 20% is allocated to the sixth grade, the rest will be distributed to the third, fourth, and fifth grades according to the ratio of 3:4:5, and the fourth grade will get 40 books. How many books are there in this batch?
一批书，分给六年级20%后，剩下的按3：4：5的比例分给三、四、五年级，四年级分得40本，这批图书共有多少本？
<gadget id="calculator">3 + 4 + 5</gadget>
<output>12</output>

<gadget id="calculator">4 / 12</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">40 / (1/3) / (4/5)</gadget>
<output>150</output>

<result>150</result>