Xiao Ming reads a book. A few days later, I found: what I have read: the rest = 1: 4, and after reading 15 pages, I found: what I have read: the rest = 1: 3. So how many pages are there in total?
小明看一本书．几天后发现：已看的：剩下的=1：4，再看15页后又发现：已看的：剩下的=1：3．那么全书一共多少页？
<gadget id="calculator">1 + 3</gadget>
<output>4</output>

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

<gadget id="calculator">1 + 4</gadget>
<output>5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">15 / (1/20)</gadget>
<output>300</output>

<result>300</result>