The bookstore sells a popular science book, a pack of 4 books, priced at 12 yuan; unpacked and retailed for 4 yuan each, if you buy 7 such books, at least how much yuan will you have to pay.
书店出售一种科普读物，一包4本，售价12元；拆包零售每本4元，如果买7本这样的书，至少要付多少元．
<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">12 + 12</gadget>
<output>24</output>

<result>24</result>