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When I update my system, I get the following errors:
Fetched 316 B in 22s (14 B/s)
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6AF0E1940624A220
Please someone suggest me a solution to this.