I added 'http://cran.at.r-project.org/' in my source.list and package manager failed to open, I am in south africa

I was trying to update my system but had the error below:
Failed to fetch http://cran.ru.ac.za/dists/bin/linux/ubuntu/lucid//binary-amd64/Packages.gz 404 Not Found
Some index files failed to download, they have been ignored, or old ones used instead.
Then I replaced http://cran.ru.ac.za with http://cran.at.r-project.org. The error below occurred:

E: Type 'http://cran.at.r-project.org/' is not known on line 7 in source list /etc/apt/sources.list
E: The list of sources could not be read.
Go to the repository dialog to correct the problem.
E: _cache->open() failed, please report.

I have tried to edit that again by entering http://cran.ru.ac.za since I am in south africa, but the error persist.