Xiao Ming reads 20 pages of a 255-page book in 4 days. According to this calculation, how many days will it take for Xiao Ming to finish reading this book?
小明看一本255页的书4天看了20页．照这样计算，小明看完这本书共需多少天？
<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">255 / 5</gadget>
<output>51</output>

<result>51</result>