6. (10 points) As shown in the figure, 75 small squares form a $15 \times 5$ pattern. Some of the small squares have already been shaded. Now, we need to continue shading some of the blank small squares to ensure that in any $2 \times 2$ square, the number of shaded small squares is more than half. What is the minimum number of additional small squares that need to be shaded? $\qquad$
【Analysis】This question actually requires that no two blank cells are adjacent (i.e., no two blank cells share a common vertex). The blank cells can be divided into 7 non-adjacent blocks as shown in the following figure. Among them, blocks $D$ and $G$ are $5 \times 1$, and each block must have at least 2 cells painted. Similarly, blocks $C$ and $F$ must each have at least 1 cell painted. The other filling methods are similar.
【Solution】Solution:
According to the painting method shown in the figure, a total of 17 cells need to be painted.
Therefore, the answer to this question is 17.