When you type: sudo apt-get update you get the error info:
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Fetched 32.6 MB in 11s (2,824 kB/s) W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/main/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/universe/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/main/binary-i386/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/universe/binary-i386/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-backports/main/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-backports/main/binary-i386/Packages Hash Sum mismatch
E: Some index files failed to download. They have been ignored, or old ones used instead.
Solution
Type this command and then type: (sudo apt-get update)
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sudo sed -i -re 's/\w+\.archive\.ubuntu\.com/archive.ubuntu.com/g' /etc/apt/sources.list