Obviously read 15% of a book every day. After 5 days, there are still 50 pages left, which is how much of the whole book.
明明每天读一本书的15%，5天后，还剩50页，是全书的多少．
<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/20) * 5</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 - (3/4)</gadget>
<output>1/4 = around 0.25</output>

<result>1/4 = around 0.25</result>