Let the total pages are 'n'. We have
1 + 2 + 3 + .........+n = 138
⇒ n(n+1) = 276
n (n+1) = 276
The square nearest to 276 is 289.This is the square of 17. Hence, we will take n= 17.
Therefore, sum of the pages= n(n+1)/2 = 17*18/2=153
Missing number = 153-138 =15.
Now, we have to think of 2 consecutive numbers whose sum is 15.
x+x+1=15 ⇒ ×= 7, 8