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To determine how old Evan is at a height of 30 inches, we should consider the typical growth patterns of infants and young children. In the first year of life, growth is rapid. On average, infants grow about 10 inches in the first 6 months and approximately another 5 inches during the second half of the first year.

If Evan was 20 inches at birth, the average expected height at 6 months would typically be around 30 inches since, as previously noted, a baby often gains close to 10 inches during that period. This growth rate aligns with the answer indicating Evan is approximately 12 months old when he reaches 30 inches.

At 12 months, children typically measure between 28 to 32 inches, which supports the conclusion that Evan would be around 12 months of age. Further, growth slows down after the first year, which suggests that it is less probable he would reach 30 inches by 6 months but more likely to do so by the time he is a year old. Therefore, given these growth standards and patterns, the answer indicating he is 12 months old at a height of 30 inches is substantiated.