Honghong reads a book. She reads 30% of the whole book on the first day, and then reads 20 pages. She reads as much as the rest. How many pages does this book have?
红红看一本书，第一天看了全书的30%，再看20页，已看的与剩下的一样多，这本书有多少页？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

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

<gadget id="calculator">20 / (1/5)</gadget>
<output>100</output>

<result>100</result>