Read 40% of a book in the first week, the remaining half in the second week, and the rest in the third week. It is known that I read 90 pages in the third week, how many pages are there in this book?
一本书第一周看了40%，第二周看了剩下的一半，第三周看完了剩下的全部．已知第三周看了90页，这本书一共有多少页？
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">90 / (1/2)</gadget>
<output>180</output>

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">180 / (3/5)</gadget>
<output>300</output>

<result>300</result>