I read a book (4/5), and there are 15 pages left. How many pages are there in this book?
一本书看了(4/5)，还剩15页．这本书一共多少页．
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">15 / (1/5)</gadget>
<output>75</output>

<result>75</result>