How to deactivate num lock on Xubuntu on MacBook?

I just tried to resurrect an old MacBook by installing Xubuntu on it. However, I face a severe problem. Every time I start Xubuntu the num lock is on and there is no key on the MacBook to deactivate it. I have no idea why num lock is on at start. The keyboard works fine on Mac OS X. And sometimes after suspend on Xubuntu, the num lock gets deactivated for no apparent reason and I can use the machine until the next restart. Is there a way to disable num lock or so to solve the problem? I have already tried to update the system to the latest version. Edit: This question is particularly about Xubuntu. And the answer to the suggested duplicate differs from the selected answer.