jan@jan ~ $ cat /etc/udev/rules.d/25-usbkeyboard.rules 
ACTION=="add", SYSFS{idProduct}=="1003", SYSFS{idVendor}=="05ac", RUN+="/usr/bin/numlockx on"
ACTION=="remove", SYSFS{idProduct}=="1003", SYSFS{idVendor}=="05ac", RUN+="/usr/bin/numlockx off"