Numbers = 3, 5, 8, 11, 14, 18, 22, 10, 9 and x. Arithmetic Mean = 12 Formula: Arithmetic mean = Sum of the Numbers ÷ Number of terms Calculation: Sum of the given numbers = 3 + 5 + 8 + 11 + 14 + 18 + 22 + 10 + 9 + x = 100 + x Number of terms = 10 Substituting the values in the equation, 12 = (100 + x) ÷ 10 12 × 10 = 100 + x 120 = 100 + x x = 120 - 100 x = 20. Hence, the correct answer is "20".