Well I think I would be best to ignore your dig at my material...
Anyway, the question asks you to remove read and execute. Read being 4 and execute 1 gives 5, and so quite rightly you calculated that 7-5 is 2, and this is indeed the first octet of the answer. However why set the other fields to 7? The question does not ask you to change these, so changing them to 7 is wrong. This is one reason why I personally recommend the symbolic notation, which would have allowed you to change only the owner octet without having to know the other octets (chmod u-rx). See http://linuxzoo.net/page/sec_symbol.html
But if you want to use this numeric notation then leave octets 2 and 3 as they were. As the default for these is 5 for directories, chmod 255 would have served you better.
I hope this helps you.