Xiaohong reads a technical book for 3 days, and there are 121 pages left. If you read at this speed for 4 days, only (2/5) of the whole book is left, how many pages does this book have in total?
小红看一本科技书，看了3天，剩下121页．如果用这样的速度看4天，就剩下全书的(2/5)，则这本书一共有多少页．
<gadget id="calculator">2 / 5</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">(3/5) / 4</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/20) * 3</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">1 - (9/20)</gadget>
<output>11/20 = around 0.55</output>

<gadget id="calculator">121 / (11/20)</gadget>
<output>220</output>

<result>220</result>