Fish, Planes, and Flags: Answer

David and his father will place 12 fish decorations along the bottom layer of the cake.

Each of the longer sides of the bottom layer measures 15 in. Allowing $one half$ in. at each end for the flags and 2 in. between all the 1 in. decorations, they can place 4 fish along each 15 in. side like this:

(flag)		(fish)		(fish)		(fish)		(fish)		(flag)
$one half$ in.	+2 in.	+1 in.	+2 in.	+1 in.	+2 in.	+ 1 in.	+2 in.	+1 in.	+2 in.	+ $one half$ in.	= 15 in.

Each of the shorter sides of the bottom layer measures 9 in. They can place 2 fish along this short side:

(flag)		(fish)		(fish)		(flag)
$one half$ in.	+2 in.	+1 in.	+2 in.	+1 in.	+2 in.	+ $one half$ in.	= 9 in.

Altogether, they can fit 12 fish (4 + 4 + 2 + 2 = 12) along the bottom layer.

The longer side of the top layer is the same length as the shorter side of the bottom layer. David and his father can fit 4 (2 + 2) airplanes on the longer sides of the top layer.

On each 6-in. side of the top layer they can fit just 1 airplane, like this:

(flag)		(plane)		(flag)
$one half$ in.	+ 2 in.	+ 1 in. +	2 in. +	$one half$ in.	= 6 in.

Altogether, they can fit 6 airplanes (2 + 2 + 1 + 1 = 6) on the top layer.