Automatically Turn on NumLock for Ubuntu

First install numlockx

$ sudo apt-get install numlockx

Method 1
$ sudo vim /etc/rc.local
Add the following content before exit 0

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if [-x /usr/bin/numlockx ]; then
numlockx on
fi

Method 2
$ sudo vim /usr/share/lightdm/lightdm.conf.d/50-unity-greeter.conf
Add greeter-setup-script=/usr/bin/numlockx on at the end.