one shell command doesn't return result

Hello,

I would like to make a simple calculation in my makefile, but I have a strange problem.
On windows, I use the DOS command "set /A 3*100+13/2", it works on a shell and returns "306" but not in my makefile. I have make a simple makefile to test it :

--------------------------------------------------
# command to make a mathematical operation
CMD_MATH:=$(shell set /A 3*100+13/2)
# command to list files
CMD_FILES:=$(shell dir /b/a-d/-W)

all::
	@echo CMD_MATH:$(CMD_MATH)
	@echo CMD_FILES:$(CMD_FILES)
--------------------------------------------------

It returns me :

--------------------------------------------------
CMD_MATH:
CMD_FILES:Makefile Makefile~
--------------------------------------------------

One command returns correctly the result but not the command "set /A" which returns nothing.
I have GNU Make 3.81

Best regards,
Fabien