When Zhang Ming reads a book, he reads 20% of the whole book in the first week, and 1/4 of the whole book in the second week, exactly 720 pages in two weeks. How many pages does this book have?
张明读一本书，第一周读了全书的20%，第二周读了全书的(1/4)，两周正好读了720页．这本书一共有多少页？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/5) + (1/4)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">720 / (9/20)</gadget>
<output>1_600</output>

<result>1_600</result>