On the first day, he read 1/3 of the total number of pages of the book.
total → 3u read (first day) → 1u not read (first day) → 3 - 1 = 2u
On the second day, he read another 50 pages.
read (2nd day) → 1u + 50 not read (2nd day) → 2u - 50
After that, the number of pages he had read was twice the number of pages he had not read.
read (2nd day) = 2 x not read (2nd day)
1u + 50 = 2 x (2u - 50) 1u + 50 = 4u - 100
1u + 50 + 100 = 4u - 100 + 100 1u + 150 = 4u
1u - 1u + 150 = 4u - 1u 3u = 150