The school library bought 15 packs of story books and 12 packs of science books, a total of 660 books, 20 story books in each pack, how many science books are in each pack? (Solution of column equation)
学校图书馆买来15包故事书和12包科技书，共660本，每包故事书20本，每包科技书多少本？(列方程解)
<gadget id="calculator">20 * 15</gadget>
<output>300</output>

<gadget id="calculator">660 - 300</gadget>
<output>360</output>

<gadget id="calculator">360 / 12</gadget>
<output>30</output>

<result>30</result>