18. Each of the numbers $1$ to $9$ is to be placed in a different cell of the grid shown so that the sum of the three numbers in each row is $15$.
Also, the sum of the two numbers in each shaded column is to be $15$.
How many choices are there for the number to be placed in the central cell indicated by * ?
![](https://assets.website-files.com/5e0b2441743608ae43ef4c83/621cbac521d7d133baea53a3_KF3a-1610467806.png)