Como resolver os problemas em “sudo apt-get update” [duplicado]

Question

Aqui está o meu arquivo /etc/apt/source.list . Eu recebo muitos problemas na saída de sudo apt-get update :

$ sudo apt-get update
[sudo] password for tim: 
Err http://cran.cnr.Berkeley.edu precise/ Release.gpg                          
  Could not connect to cran.cnr.Berkeley.edu:80 (169.229.201.201). - connect (111: Connection refused)
Err http://ppa.launchpad.net precise/main Sources        
  404  Not Found
Err http://ppa.launchpad.net precise/main i386 Packages  
  404  Not Found

W: Failed to fetch http://cran.cnr.Berkeley.edu/bin/linux/ubuntu/precise/Release.gpg  Could not connect to cran.cnr.Berkeley.edu:80 (169.229.201.201). - connect (111: Connection refused)

W: Failed to fetch http://ppa.launchpad.net/venerix/blug/ubuntu/dists/precise/main/source/Sources  404  Not Found

W: Failed to fetch http://ppa.launchpad.net/venerix/blug/ubuntu/dists/precise/main/binary-i386/Packages  404  Not Found

W: Some index files failed to download. They have been ignored, or old ones used instead.

Eu tenho vários erros no final da saída. Como posso resolver os problemas?

Obrigado!