6. (10 points) As shown in the figure, in each cell of a $5 \times 5$ grid, fill in a natural number (natural numbers include 0), such that the average of the four numbers in each $2 \times 2$ subgrid is 100. Then, the minimum value of the average of the 25 natural numbers filled in the entire $5 \times 5$ grid is . $\qquad$
【Analysis】Take the 4 $2 \times 2$ squares from the top-left, bottom-left, top-right, and bottom-right corners. The 16 small squares they contain do not overlap with each other, so the sum of the numbers in this table will not be less than $400 \times 4=1600$, and the average will not be less than 1600 $\div 25=64$. Constructing a shape with an average of 64 can solve the problem.

【Solution】Since the average of the four numbers in each $2 \times 2$ square is 100, the sum of the four numbers is 400. Taking the 4 $2 \times 2$ squares from the top-left, bottom-left, top-right, and bottom-right corners, the 16 small squares they contain do not overlap with each other, so the sum of the numbers in this table will not be less than $400 \times 4=1600$, and the average will not be less than $1600 \div 25=64$. When the numbers are filled as shown in the figure, it meets the requirements, so the minimum average value of the 25 natural numbers filled in the $5 \times 5$ table is 64.

Therefore, the answer is 64.