It is true that there are similar questions in the forum... But as I installed Ubuntu under Windows 7 long time ago, I do not remember how I did it or whether I used Wubi, which is discussed in many similar questions.
So the situation is, I had Windows 7 installed on the laptop in the very beginning, then I installed Ubuntu so that this is a dual system (once again, i could not remember how I did it).
Today when I use "Disk Usage Analyzer" to scan the disks. It says:
Total filesystem capacity: 303.2 GB (used: 162.2 GB available: 141.0 GB)
And the size of root is 161.1 GB whose usage is 100%. It seems that the files under my Windows are all counted in this 161.GB. So now the question is how to make use of the rest 141.0 GB, especially extend Ubuntu space with that.
Could anyone help? Thank you very much.
As suggested, here is some images from Disk Utility: http://imgur.com/a/CG06S#hGaZl and images from Disk Usage Analyzer http://imgur.com/SVpaV , the folder under /media is mounted from Windows (so both Windows and Ubuntu could share it)