The number of bottles in a full box is 12.
Step-by-step explanation
Suppose, the number of bottles in a full box is

So, the number of bottles in three full boxes

Now, two more boxes are added, both of which have one fewer bottle than the other three. So, the number of bottles in these two boxes

As there were 9 bottles initially and now there are 67 bottles in the fridge, so the equation will be....

Thus, the number of bottles in a full box is 12.