If Corbin read 4/7 of his book, he has 3/7 of the book left to read.
This means that 3/7 of the book is equal to 42 pages.
42 ÷ 3 = 14
so 1/7 of the book is equal to 14 pages.
Since Corbin has read 4/7 of the book, 14 should be multiplied by 4.
14 × 4 = 56
Corbin has read 56 pages