If the chronological age of a child is 12 years and 6 months, and his mental age is 18 years, then…

Given Chronological Age ($\text{CA}$) = 12 years 6 months = $12.5$ years, and Mental Age ($\text{MA}$) = $18$ years. Using the formula $\text{IQ} = \frac{\text{MA}}{\text{CA}} \times 100 = \frac{18}{12.5} \times 100 = 144$. According to Lewis Terman's classification table, an IQ of 140 or above falls in the 'Gifted' (or 'Genius') category.