The school spent 308 yuan to buy 30 copies of "Stories of Idioms" and "World of Mathematics", 8 yuan each for "Stories of Idioms", and 12 yuan for each copy of "World of Mathematics". Please do the math, how many copies of "Mathematics World" are there?
学校用308元买来《成语故事》和《数学大世界》共30本，《成语故事》每本8元，《数学大世界》每本12元．请你算一算，《数学大世界》有多少本？
<gadget id="calculator">12 * 30</gadget>
<output>360</output>

<gadget id="calculator">360 - 308</gadget>
<output>52</output>

<gadget id="calculator">12 - 8</gadget>
<output>4</output>

<gadget id="calculator">52 / 4</gadget>
<output>13</output>

<gadget id="calculator">30 - 13</gadget>
<output>17</output>

<result>17</result>