software installation - Error during apt-get update

While running apt-get i get the following error :-

W: GPG error: https://dl.bintray.com  Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 99E82A75642AC823
W: Failed to fetch http://ppa.launchpad.net/picaso/octave/ubuntu/dists/trusty/main/binary-amd64/Packages  404  Not Found
W: Failed to fetch http://ppa.launchpad.net/picaso/octave/ubuntu/dists/trusty/main/binary-i386/Packages  404  Not Found
W: Failed to fetch http://ppa.launchpad.net/samrog131/ppa/ubuntu/dists/trusty/main/binary-amd64/Packages  404  Not Found
W: Failed to fetch http://ppa.launchpad.net/samrog131/ppa/ubuntu/dists/trusty/main/binary-i386/Packages  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.

I have tried a number of solutions each of which failed. I can't install any software using sudo apt-get update in their script now :(
I have already tried system updates, trying to contact the keyserver (which always returns a timeout)

Typically running the following should work:

 sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 99E82A75642AC823
 sudo apt-get update

In this case, I don't think this will work because:

1. There is no trusty packages for octave as you can see from here




2. User samrog131 no longer maintains a PPA as you can see from here

It is better to remove these PPAs from your sources list. Read an excellent answer from How can I fix a 404 Error when using a PPA or updating my package lists? to know how to do that.