A bookshelf with upper and lower floors, if 20 books are taken from the lower layer and placed in the upper layer, then the number of books on the upper layer is less than the number of books on the lower layer (1/4). It is known that there were 100 books in the upper layer, how many books were there in the lower layer?
一个上下两层的书架，若从下层取出20本书放入上层，则上层的书比下层的书少(1/4)．已知上层原来有100本书，下层原来有多少本书？
<gadget id="calculator">100 + 20</gadget>
<output>120</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">120 / (3/4)</gadget>
<output>160</output>

<gadget id="calculator">160 + 20</gadget>
<output>180</output>

<result>180</result>