There are 180 volumes of books, which are assigned to Class 1 and Class 2 according to the ratio of 4:5. How many books are there between the two classes?
有图书180册，按4：5分给一班和二班，两班相差多少册．
<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">180 / 9</gadget>
<output>20</output>

<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">20 * 1</gadget>
<output>20</output>

<result>20</result>