The fraction, 1/12, can be written as 1/4 * 1/3. Since we know that 1/3 = 0.333333... and it doesn't terminate, we know that 1/12 doesn't terminate. Similarly 2/9 = 2/3*1/3 and 2/3 = 2*1/3, and neither of these fractions terminate. But, when dividing 3/10, you get 0.3, which terminates.
