GPG error: http://us.archive.ubuntu.com/ubuntu precise-security InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 code example

Example: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6494C6D6997C215E