Xiao Ming reads a book, reads 30% of the whole book on the first day, and reads 21 pages less on the second day than on the first day, and still has not read half of it. How many pages does this book have?
小明看一本书，第一天看了全书的30%，第二天比第一天少看21页，这时还有一半没有看．这本书有多少页？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/2) - (3/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(3/10) - (1/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">21 / (1/10)</gadget>
<output>210</output>

<result>210</result>