Xiao Ming borrows a book. If he reads 25 pages a day on average, he can return it 12 days in advance. If he reads 20 pages a day on average, he can return it 8 days in advance. How many pages does this book have?
小明借一本书，如果平均每天看25页，可以提前12天交还，如果平均每天看20页，可以提前8天交还，这本书共有多少页？
<gadget id="calculator">12 - 8</gadget>
<output>4</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 25</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">(1/20) - (1/25)</gadget>
<output>1/100 = around 0.01</output>

<gadget id="calculator">4 / (1/100)</gadget>
<output>400</output>

<result>400</result>