$9 \cdot 61$ Given an infinitely large grid paper where each cell is colored either red or blue, such that in any $2 \times 3$ rectangle of cells, there are exactly two red cells. How many red cells are contained in a rectangle of $9 \times$ 11 cells?
[Solution] We use $R$ and $B$ to represent red and blue cells, respectively. If in a $1 \times 3$ rectangle there are 2 red cells, then the two red cells are either adjacent or separated by 1 cell (see figure). In either case, it must lead to a $2 \times 3$ rectangle having at most 1 red cell, which contradicts the given condition. Therefore, each $1 \times 3$ rectangle must contain exactly 1 red cell, hence there are 33 red cells in the $9 \times 11$ rectangle.