W: GPG error: http://ppa.launchpad.net lucid Release: The following signatures couldn

When running

sudo apt-get update

I was getting the following errors:

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 E635A714505A2DD1
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 1DABDBB4CEC06767
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: http://download.virtualbox.org lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 54422A4B98AB5139
W: GPG error: http://deb.opera.com stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F9A2F76A9D1A0061

The solution is simple.Take the first key you have problem with, for example E635A714505A2DD1, take the last 8 symbols (in this case - 505A2DD1) and run the following in the terminal:

gpg --keyserver keyserver.ubuntu.com --recv 505A2DD1
gpg --export --armor 505A2DD1 | sudo apt-key add -

Then repeat this step with every problematic key.