Since, the repetition is not allowed, we cannot form a four- digit or a higher digit number. We have the following possible cases: One-digit number: There are three possibilities (3, 5, 7) for the single digit. ⇒ 3 possibilities. Two-digit number: There are three possibilities for one of the digits, and corresponding to each possibility, the other digit will have two possibilities. ⇒ 3 × 2 = 6 possibilities. Three-digit number: There are three possibilities for one of the digits, and then two possibilities for the second digit and then only a single possibility for the third digit. ⇒ 3 × 2 ×1 = 6 possibilities. ∴ The total number of possible numbers = 3 + 6 + 6 = 15.