Xiao Ming saw 5/(4+5)=5/9.

Xiao Ming went on to read page 27, which happened to be an unfinished 2: 1.

Xiao Ming saw 2/(2+ 1)=2/3.

Then these 27 pages belong to the whole book: 2/3-5/9= 1/9.

So this book has 27 pages (1/9) = 243 pages.