A bookshelf has four floors, and the number of books stored on the first floor accounts for 20% of the total. If 10 books are placed on the first floor from the third floor, the number of books stored on the four floors is equal. How many books are stored in this bookshelf?
一个书架共有四层，第一层存数量占总数的20%，如果从第三层拿10本放到第一层，则四层的存书本数分别相等，这个书架共存书多少本．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">10 / (1/20)</gadget>
<output>200</output>

<result>200</result>