I'm facing this when I fresh install Debian 7.

No matter what I trying to install, it doesn't works.

Keep showing E: Unable to locate package ...

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root@debian:~# apt-get install git
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package git
root@debian:~# apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

Then I found the solution. Edit the source list

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# vi /etc/apt/sources.list

to this content

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deb http://ftp.at.debian.org/debian wheezy main contrib
# Line commented out by installer because it failed to verify:
deb http://security.debian.org/ wheezy/updates main
# Line commented out by installer because it failed to verify:
deb-src http://security.debian.org/ wheezy/updates main

Then run

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# apt-get update
# apt-get upgrade

Now can install other packages

References: