Boxes are 6 cm × 18 cm and have an aspect ratio of 1:3. In terms of the minimum dimension (6 cm), the shelf area is 10 5/6 × 5 5/6, so 10 × 5 = 50 times the square of the minimum dimension.
Since the shelf space is occupied in multiples of 3 such squares, there can be ⌊50/3⌋ = 16 boxes on the shelf.
_____
There will be 5 cm of unused space in each direction, along with a 12 cm × 6 cm rectangle that cannot be filled with a cereal box.
