Ubuntu update error

When I ran "sudo-apt update" command in Terminal, I get the following error.

commandrine@bridge:~$ sudo apt-get update
[sudo] password for commandrine:
Hit http://sg.archive.ubuntu.com lucid Release.gpg
Ign http://sg.archive.ubuntu.com/ubuntu/ lucid/main Translation-en_SG
Ign http://sg.archive.ubuntu.com/ubuntu/ lucid/restricted Translation-en_SG
Ign http://sg.archive.ubuntu.com/ubuntu/ lucid/universe Translation-en_SG
Ign http://sg.archive.ubuntu.com/ubuntu/ lucid/multiverse Translation-en_SG
Hit http://sg.archive.ubuntu.com lucid-updates Release.gpg
Ign http://sg.archive.ubuntu.com/ubuntu/ lucid-updates/main Translation-en_SG
Ign http://sg.archive.ubuntu.com/ubuntu/ lucid-updates/restricted Translation-en_SG
Ign http://sg.archive.ubuntu.com/ubuntu/ lucid-updates/universe Translation-en_SG
Ign http://sg.archive.ubuntu.com/ubuntu/ lucid-updates/multiverse Translation-en_SG
Hit http://sg.archive.ubuntu.com lucid Release
Hit http://sg.archive.ubuntu.com lucid-updates Release
Hit http://sg.archive.ubuntu.com lucid/main Packages
Hit http://sg.archive.ubuntu.com lucid/restricted Packages
Hit http://sg.archive.ubuntu.com lucid/main Sources
Hit http://sg.archive.ubuntu.com lucid/restricted Sources
Hit http://sg.archive.ubuntu.com lucid/universe Packages
Hit http://sg.archive.ubuntu.com lucid/universe Sources
Hit http://sg.archive.ubuntu.com lucid/multiverse Packages
Hit http://sg.archive.ubuntu.com lucid/multiverse Sources
Hit http://sg.archive.ubuntu.com lucid-updates/main Packages
Hit http://sg.archive.ubuntu.com lucid-updates/restricted Packages
Hit http://sg.archive.ubuntu.com lucid-updates/main Sources
Hit http://sg.archive.ubuntu.com lucid-updates/restricted Sources
Hit http://sg.archive.ubuntu.com lucid-updates/universe Packages
Hit http://sg.archive.ubuntu.com lucid-updates/universe Sources
Hit http://sg.archive.ubuntu.com lucid-updates/multiverse Packages
Hit http://sg.archive.ubuntu.com lucid-updates/multiverse Sources
Hit http://ppa.launchpad.net lucid Release.gpg
Hit http://security.ubuntu.com lucid-security Release.gpg
Ign http://ppa.launchpad.net/docky-core/ppa/ubuntu/ lucid/main Translation-en_SG
Ign http://security.ubuntu.com/ubuntu/ lucid-security/main Translation-en_SG
Hit http://ppa.launchpad.net lucid Release.gpg
Ign http://security.ubuntu.com/ubuntu/ lucid-security/restricted Translation-en_SG
Ign http://ppa.launchpad.net/docky-core/stable/ubuntu/ lucid/main Translation-en_SG
Ign http://security.ubuntu.com/ubuntu/ lucid-security/universe Translation-en_SG
Get:1 http://ppa.launchpad.net lucid Release.gpg [316B]
Ign http://ppa.launchpad.net/moonlight-team/pinta/ubuntu/ lucid/main Translation-en_SG
Ign http://security.ubuntu.com/ubuntu/ lucid-security/multiverse Translation-en_SG
Get:2 http://ppa.launchpad.net lucid Release.gpg [316B]
Get:3 http://security.ubuntu.com lucid-security Release [44.7kB]
Ign http://ppa.launchpad.net/mozillateam/firefox-stable/ubuntu/ lucid/main Translation-en_SG
Hit http://ppa.launchpad.net lucid Release.gpg
Ign http://ppa.launchpad.net/pidgin-developers/ppa/ubuntu/ lucid/main Translation-en_SG
Hit http://security.ubuntu.com lucid-security/main Packages
Get:4 http://ppa.launchpad.net lucid Release.gpg [316B]
Ign http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu/ lucid/main Translation-en_SG
Hit http://security.ubuntu.com lucid-security/restricted Packages
Hit http://ppa.launchpad.net lucid Release
Hit http://security.ubuntu.com lucid-security/main Sources
Get:5 http://security.ubuntu.com lucid-security/restricted Sources [14B]
Hit http://ppa.launchpad.net lucid Release
Hit http://security.ubuntu.com lucid-security/universe Packages
Get:6 http://ppa.launchpad.net lucid Release [57.3kB]
Ign http://ppa.launchpad.net lucid Release
Get:7 http://ppa.launchpad.net lucid Release [14.0kB]
Ign http://ppa.launchpad.net lucid Release
Hit http://security.ubuntu.com lucid-security/universe Sources
Get:8 http://ppa.launchpad.net lucid Release [14.0kB]
Hit http://security.ubuntu.com lucid-security/multiverse Packages
Hit http://security.ubuntu.com lucid-security/multiverse Sources
Get:9 http://ppa.launchpad.net lucid Release [14.0kB]
Ign http://ppa.launchpad.net lucid Release
Hit http://ppa.launchpad.net lucid/main Packages
Get:10 http://ppa.launchpad.net lucid/main Packages [1,506B]
Get:11 http://ppa.launchpad.net lucid/main Packages [752B]
Hit http://ppa.launchpad.net lucid/main Packages
Hit http://ppa.launchpad.net lucid/main Packages
Ign http://ppa.launchpad.net lucid/main Sources
Hit http://ppa.launchpad.net lucid/main Packages
Ign http://ppa.launchpad.net lucid/main Sources
Err http://ppa.launchpad.net lucid/main Sources
416 Requested Range Not Satisfiable 20003
Fetched 61.8kB in 22s (2,794B/s)
W: GPG error: http://ppa.launchpad.net lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3AD52A40B98E84D3
W: GPG error: http://ppa.launchpad.net lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9BDB3D89CE49EC21
W: GPG error: http://ppa.launchpad.net lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A6DCF7707EBC211F
W: Failed to fetch http://ppa.launchpad.net/pidgin-developers/ppa/ubuntu/dists/lucid/main/source/Sources.gz 416 Requested Range Not Satisfiable 20003

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

Turning to the Ubuntu Forum, I get advise on running the following commands to rectify the missing keys issue.

gpg --keyserver keyserver.ubuntu.com --recv 3AD52A40B98E84D3

gpg --export --armor 3AD52A40B98E84D3 | sudo apt-key add -

gpg --keyserver keyserver.ubuntu.com --recv 9BDB3D89CE49EC21

gpg --export --armor 9BDB3D89CE49EC21 | sudo apt-key add -

gpg --keyserver keyserver.ubuntu.com --recv A6DCF7707EBC211F

gpg --export --armor A6DCF7707EBC211F | sudo apt-key add -

I encountered 2 keys that still generate update errors. Heading back to the Forum, I get the following steps to rectify the issue.

sudo -i

apt-get clean

cd /var/lib/apt

mv lists lists.old

mkdir -p lists/partial

apt-get clean

apt-get update

Problem solved.