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This question already has an answer here:

When update crashes I got this message:

W:Failed to fetch http://cran.at.r-project.org/dists/precise-backports/main/source/Sources  404  Not Found
E:Some index files failed to download. They have been ignored, or old ones used instead.
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marked as duplicate by Jorge Castro, Luis Alvarado Mar 28 '13 at 15:05

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

1  
Looks like an external repository you've added is to blame. Remove it, and updating should work. – mikewhatever Mar 19 '13 at 23:11
    
This might be a good way to deal with this: askubuntu.com/questions/39922/… – Mordoc Mar 20 '13 at 1:25
    
This isn't a PPA, it's just an external repository, but the answer to that question will fix this here too. – Jorge Castro Mar 28 '13 at 13:09

I think your link is wrong, try this one

deb  http://cran.at.r-project.org/bin/linux/ubuntu precise/
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