Put (1/5) of the number of books on the upper layer into the lower layer. Since the number of books on the upper and lower layers is the same, the number of books on the lower layer is equal to the number of books on the upper layer.
把上层书本数的(1/5)放到下层，由上下两层本数相同，下层书的本数原来是上层的多少．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5) - (1/5)</gadget>
<output>3/5 = around 0.6</output>

<result>3/5 = around 0.6</result>