To add these two fractions, you first need to find a common denominator. In this case, the common denominator is 4.

Converting 1/2 to have a denominator of 4:
1/2 x 2/2 = 2/4

Now, you can add the fractions:
2/4 + 5/4 = 7/4

Therefore, 3:1 1/2 + 5:1 1/4 = 8:3 1/4