The school bought a total of 30 story books and science books, each story book was 15 yuan, and each science book was 25 yuan. The total cost was 550 yuan. How many story books did you buy?
学校新买故事书和科技书共30本，每本故事书15元，每本科技书25元，共花550元，买故事书多少本？
<gadget id="calculator">750 - 550</gadget>
<output>200</output>

<gadget id="calculator">25 - 15</gadget>
<output>10</output>

<gadget id="calculator">200 / 10</gadget>
<output>20</output>

<result>20</result>