Xiao Ming reads a book. He reads 20% of the whole book on the first day, and 30% of the whole book on the second day. There are still 25 pages left. How many pages does this book have? (Solve by two methods of arithmetic and equation)
小明看一本书，第一天看了全书的20%，第二天看了全书的30%，还有25页没看，这本书一共有多少页？(用算术和方程两种方法解答)
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">1 - (1/5) - (3/10)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">25 / (1/2)</gadget>
<output>50</output>

<result>50</result>