Xiao Ming reads a book. He reads (1/4) of the whole book on the first day and (2/5) of the whole book on the second day. He reads 21 pages more on the second day than on the first day. How many pages.
小明看一本书，第一天看了全书的(1/4)，第二天看了全书的(2/5)，第二天比第一天多看了21页，这本书一共有多少页．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

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

<gadget id="calculator">(2/5) - (1/4)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">21 / (3/20)</gadget>
<output>140</output>

<result>140</result>