Let the left page be page number N
Then the right page is page number N+1
The sum of the pages is N + (N+1)
So the equation is
N + (N+1) = 157
N + N + 1 = 157
2N + 1 = 157
2N = 156
N = 78
So the page on the left is page 78 and
the page on the right is 79.
Edwin