when rolling two 6 sided number cubes, what are the chances the sum of the roll will be a multiple of 3

There are 3*3=9 total possibilities for the two dice, which makes the probability of getting a multiple of three 3/9=1/3. Since there are 6*6=36 total dice rolls and 1/3 of those are a multiple of three, the number which are divisible by three is (1/3) (36) = 12.