Clear setuid permission using numeric mode

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http://serverfault.com – On an Ubuntu Linux I have a directory with the setuid bit set (drwsr-xr-x) which I want to unset. Neither chmod 755 nor chmod 0755 nor chmod 00755 (I though maybe the first 0 is interpreted as just "this is octal") cleared the setuid bit. However, chmod u-s did. What is the correct numeric mode to clear the setuid bit? (HowTos)