Zhang Liang reads an extra-curricular book. He read 30% of the book on the first day and 15 pages on the second day. At this time, the ratio of the number of pages read to the number of pages in the book is 7:8. How many pages does this extracurricular book have?
张亮看一本课外书，第一天看了全书的30%，第二天看了15页，这时已看页数与全书页数的比是7：8．这本课外书有多少页？
<gadget id="calculator">7 / 15</gadget>
<output>7/15 = around 0.466667</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(7/15) - (3/10)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">15 / (1/6)</gadget>
<output>90</output>

<result>90</result>