Problem with Darwin typeset -i

From the bash manpage:
typeset [-afFirtx] [-p] [name[=value]]

What I'm trying to do is add two values in hexadecimal and have the resulting number display in hexadecimal. What I get is the result displayed in decimal. For instance:

a=0x10
b=0x30
((c=a+b))
echo $c

...displays 64. The arithmatic is correct, but the format should be 0x40. I tried using typeset -i16 c, but apparently, the option -i doesn't accept parameters in Darwin, so I get this error:

-bash: typeset: -1: invalid option
typeset: usage: typeset [-afFirtx] [-p] name[=value] ...

Can anyone help the newbie with a different solution? How can I get the result to display in hexadecimal format?

printf "0x%x" $c

man printf

Thanks, but now I really feel newbie-ish! The only consolation I have is that I just looked up the manpage before I came back here. I didn't figure it out, though, so...thanks.