Xiaohong reads 25% of a book in the morning and 21 pages in the afternoon. At this time, the ratio of the number of pages read to the number of unread pages is 3:2. How many pages does this book have?
小红读一本书，上午读了25%，下午读了21页，这时已读的页数与未读的页数之比是3：2，这本书共有多少页？
<gadget id="calculator">3 + 2</gadget>
<output>5</output>

<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(3/5) - (1/4)</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">21 / (7/20)</gadget>
<output>60</output>

<result>60</result>