2nd page = 1st page's number + 1
3rd page = 1st page number + 2
4th page = 1st page's number + 3
.
.
.
9th page = 1st page's number + 8
10th page = 1st page's number + 9
11th page = 1st page's number + 10

total = 374

Subtract all the differences from 374.


374-10-9-8-7-6-5-4-3-2-1 = 319

319 is the same as 11 of the 1st page's number added together

319 ÷ 11 = 29

The 1st page of Chapter 3 is page 29.

Or even better: start from the middle page.

There must be 5 pages before and 5 pages after.

The sum of the page numbers is 11 x the middle page number.

This even-out method works directly for odd numbered of pages. For even numbered of pages, adjustment is needed.

Ok, noted. Thank you very much.