To subtract the fractions, we first need to find a common denominator.

The common denominator for 30 and 12 is 60.

So, we convert 3 7/30 and 1 5/12 to have a denominator of 60:

3 7/30 = 3 60/30 + 72/30 = 180/30 + 14/30 = 194/30
1 5/12 = 1 60/12 + 55/12 = 60/12 + 25/12 = 85/12

Now, we can subtract the two fractions:

194/30 - 85/12

First, convert 85/12 to have a denominator of 30:

85/12 = 852.5 / 122.5 = 212.5 / 30

Now, we can subtract the fractions:

(194/30) - (212.5/30) = -18.5/30 = -37/60

Therefore, 3 7/30 - 1 5/12 is equal to -18 1/16.