To add these fractions together, we first need to find a common denominator.

First, we convert the mixed numbers into improper fractions:

18 9/20 = 18 + 9/20 = 18*20/20 + 9/20 = 360/20 + 9/20 = 369/20

6 7/30 = 6 + 7/30 = 6*30/30 + 7/30 = 180/30 + 7/30 = 187/30

4 11/12 = 4 + 11/12 = 4*12/12 + 11/12 = 48/12 + 11/12 = 59/12

Now, we need to find the least common multiple of 20, 30, and 12, which is 60.

Now, rewrite the fractions with a denominator of 60:

369/20 = 1107/60

187/30 = 1249/60

59/12 = 295/60

Now, we can add the fractions:

1107/60 + 1249/60 + 295/60 = (1107 + 1249 + 295)/60 = 2651/60

Thus, 18 9/20 + 6 7/30 + 4 11/12 = 2651/60 or 44 11/60.