read a book (4/7), and there are 21 pages left. How many pages does this book have? How much is the correct column
一本书看了(4/7)，还剩21页，这本书有多少页？正确列式多少
<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">1 - (4/7)</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">21 / (3/7)</gadget>
<output>49</output>

<result>49</result>