Which of these numbers is not divisible by 3?

Key insight: A number is divisible by 3 if the sum of its digits is divisible by 3.

• 121: 1 + 2 + 1 = 4, which is not divisible by 3. So 121 is not divisible by 3.

• 243: 2 + 4 + 3 = 9, which is divisible by 3. So 243 is divisible by 3.

• 321: 3 + 2 + 1 = 6, which is divisible by 3. So 321 is divisible by 3.

• 111: 1 + 1 + 1 = 3, which is divisible by 3. So 111 is divisible by 3.

Final answer: 121 is the only number not divisible by 3.