2 Answers

/24

3 7/12,5/8,3 5/6,3/4

Your denominators are 12, 8, 6, and 4.  You find the lowest common denominator by finding the number of least value that has all of those denominators as factors.  
The best way to do this is to take the LARGEST denominator (12) and list the multiples of 12:
12,  24,  36,  48,  60,  72,  84,  96,  108,  120
Next, list the multiples of the NEXT highest denominator (8), til you get one that matches
12, 24 (aha)
NOW, as if you couldn't guess, do the same thing with 6, and then 4
6, 12, 18, 24 (again)
4, 8, 12, 16, 20, 24 (and yet again)

This means the LOWEST COMMON DENOMINATOR is 24.
To figure out how much each fraction/mixed number changes to, multiply each denominator by the number that brings it to 24.
12 x 2    7/12 = 14/24
8 x 3     5/8 = 15/24
6 x 4     5/6 = 20/24
4 x 6     3/4 = 18/24