Unable to locate package ubuntu-restricted-extras and software center issue

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http://askubuntu.com – I use Lenovo G450 and just installed 11.10, but hadn't checked third party software installation at that time. Now, when i code "sudo apt-get install ubuntu-restricted-extras," i am getting this message "Unable to locate package ubuntu-restricted-extras". Also, when I try from Software center, there is "updating cache" running from long time and this selection for restricted-extras adds two more "updating cache" and "enabling component of the distribution". All in waiting mode. what should I do?? (HowTos)