In order to determine the number of page numbers, let x the number of one page and x + 1 the number of the other page.
The sum of both numbers is 381, then:
x + (x + 1) = 381
Solve the previous equation for x, as follow:
x + x + 1 = 381 simplify like terms
2x + 1 = 381 subtract 1 both sides
2x = 381 - 1
2x = 380 divide by 2 both sides
x = 380/2
x = 190
Then, the numbers are 190 + 191
