Suppose you read page X on the first day, and what you didn't read on the first day was page 5x.

The next day, after reading 6 pages, I have read x+6 pages, but I have not read 5x-6 pages. The ratio between them is 1: 4.

therefore

(x+6):(5x-6)= 1:4

solve an equation

get

X=30 pages

All pages =30+30*5= 180 pages