The sum of mary and al's age is 48. twelve years ago, mary was twice as old as al.

Let us denote the years of Mary with M and the years that Al have with A.
We know that the sum of Mary and Al's age is 48. So, we can write:
M+A=48 (1)
We also know that twelve years ago, Mary was twice as old as Al. So we can write:
M-12=2*A (2)
From the first equation (1): M=48-A
Let put this in the second equation:
48-A-12=2A
36=3A
A=12
M=48-12=36