18. (6 points) As shown in the figure, select 5 numbers from $1,2,3,4,5,6$ to fill in the empty cells in the figure, such that the number on the right is greater than the one on the left, and the number below is greater than the one above. Then there are $\qquad$ different ways to fill the cells.
【Analysis】This problem is solved based on the multiplication principle. Selecting 5 numbers from 6, there are $6 \times 5$ ways.
【Solution】Solution: Selecting 5 numbers from 6, there are:
$$
6 \times 5=30 \text { (ways); }
$$

Therefore, the answer is: 30.