Problem with the HDD of my MacBook 5,1

So I have this aluminum MacBook from 2008 with 160 HDD and I want to install ubuntu for a class at my college, but I encountered a problem...

disk utility says that there are 60 gb available

but when I'm at finder it says that there are only 40 gb available

why is this happening and what can I do to solve this problem?

have you emptied your trash?

robduckyworth said:

have you emptied your trash?

Click to expand...

yup already ;/ many times

try repairing permissions through disk utility

robduckyworth said:

try repairing permissions through disk utility

Click to expand...

what is this for?
I dont know exactly how to do this ;/

sometimes the permissions of mac osx get messed up and your computer does some strange things. you go into disk utility, select your osx startup (macintosh hd) click first aid tab, then at the bottom it says repair disk permissions. click it.