The mean (average) of 6, 9 and 18 is equal to the mean (average) of 12 and $y$. What is the value of $y$?
The mean of 6, 9 and 18 is $$\frac{6+9+18}{3}=\frac{33}{3}=11.$$Since the mean of 12 and $y$ is thus 11, then the sum of 12 and $y$ is $2(11)=22$, so $y=\boxed{10}$.