Let x = p/q be a rational number, such that the prime factorization of q is of the form 2ⁿ 5^m, where n, m are non-negative integers. Then x has a decimal expansion which terminates.

(i) Here q = 225

225 can be written as 3²×5²

Since it is in the form of 5^m, it is a terminating decimal.

(ii) Here q = 18

18 can be written as 2 × 3²

Since 3 is also there and it is not in the form of 2ⁿ5^m, it is not a terminating decimal.

(iii) Here q = 21

21 can be written as 3 × 7

Since it is not in the form of 2ⁿ5^m, it is not a terminating decimal.

(iv) Here q = 250

250 can be written as 2 × 5³

Since it is in the form of 2ⁿ5^m, it is a terminating decimal.