Xiao Ming reads a story book. On the first day, he reads (1/4) of the whole book, and on the second day (2/5), there are 21 pages left. How many pages are there in this book?
小明看一本故事书，第一天看了全书的(1/4)，第二天(2/5)，还剩21页，这本书一共多少页？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (1/4) - (2/5)</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">21 / (7/20)</gadget>
<output>60</output>

<result>60</result>