The volume of books on the upper shelf is 3 times that of the lower shelf. After the lower shelf gives 30 books to the upper shelf, the book storage on the upper shelf is 9 times that of the lower shelf. How many books are there in the lower shelf?
书架上层存书量是下层的3倍，下层给上层30本后，上层存书量是下层的9倍，原来下层有多少本书？
<gadget id="calculator">30 * 9</gadget>
<output>270</output>

<gadget id="calculator">270 + 30</gadget>
<output>300</output>

<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<gadget id="calculator">300 / 6</gadget>
<output>50</output>

<result>50</result>