A terminating decimal only happens when the fraction has a denominator with factors of 2 and/or 5. No other factors are allowed (other than 1).
Something like 1/12 has the denominator 12 with factors 2*2*3. That '3' means we won't have a terminating decimal. Similarly, the fractions 2/9 and 2/3 won't convert to a terminating decimal either.
3/10 on the other hand does convert to a terminating decimal and 3/10 = 0.3
