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