15. Fill in the circles with natural numbers from  to $_{12}$ (each number must be used exactly once), so that the sum of the 4 numbers in the circles connected by the same straight line is equal. This is called a ${ }_{6}$-order magic star, and this equal number is called the magic sum of the ${ }_{6}$-order magic star. Then, the magic sum of the 6-order magic star is $\qquad$, and continue to complete the following $6_{6}$-order magic star:

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将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。
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Note: The last sentence is a note to the translator and should not be included in the translated text. Here is the final translated text:

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15. Fill in the circles with natural numbers from  to $_{12}$ (each number must be used exactly once), so that the sum of the 4 numbers in the circles connected by the same straight line is equal. This is called a ${ }_{6}$-order magic star, and this equal number is called the magic sum of the ${ }_{6}$-order magic star. Then, the magic sum of the 6-order magic star is $\qquad$, and continue to complete the following $6_{6}$-order magic star:
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Solution: Observing the figure, there are six magic sums, these six magic sums use the numbers $1 \sim 12$ exactly twice, so the total of the six magic sums is $(1+12) \times 12 \div 2 \times 2=156$, thus the magic sum is $156 \div 6=26$.